Side note--I was wrong in my reading of Contagion. The target needs to fail 3 saves before the negative effects kick in.
(True Polymorph, I'll grant, might be. Not so impressed as you are by Foresight/Wish/Prismatic Wall... you realise that Foresight only grants disadvantage for enemies specifically targeting the 1 target, yes?). I don't really think this is an advantage Wizard(/Sorcerer/Bard) enjoys as convincingly at lower levels.
The big thing about most of these is that they break the concentration economy. True Polymorph requires concentration for one hour (and then it's permanent until dispelled). Foresight does not require concentration.
I'll admit it depends a little on the DM, however. Apparently the official guideline is for the DM to give 8 encounters per day, and 4 short rests. Meteor Swarm wins one encounter. Foresight helps you in all 8 encounters. Stick it on the Rogue, now they don't need to worry about being squishy, always get sneak attack, they can feel comfortable relying on their evasion for spells, they are at almost no risk from traps (very likely to succeed on the skill check, and advantage to the saving throw if they fail).
And you know, Wish is really good. Like...ignore material components of spells, and the spell always takes 1 action even if it's replicating a spell that normally takes 12 hours to cast. So...walk up to the super powerful bad guy, and use Wish to duplicate the effects of the spell Simulacrum. Congrats, you now have a copy of the super powerful badguy who is friendly to you and obeys your commands!
And... the other spells you list don't change my point! My point was that Wizard (and Sorcerer and Bard, yes) specifically spike up a lot with that spell, and others like it, if you feel any are as good
Well yes. This is why I was harsh on the level 20 evaluation of casters who don't get this power spike. (Hence...harsh on Cleric. Fairly harsh on Warlock because while they have perfectly fine options at 9 they only get to pick one and can never change it. And...while Land Druid has generally decent spells, still putting them below casters with stronger spell lists like Bard and Wizard. Land Druid, incidentally, could probably be argued to have a better spell list/spell knowing mechanic than Sorcerer, but I think their lists are close enough that metamagic wins).
It's the most overtly powerful, and the spell which I've generally seen the most hype for online. It's also a huge upgrade over the most damaging L8 spell, which is around 50 points for AoE or 96 ST.
OK, want to come back to this for a bit. 96 would be...Disintegrate cast out of a level 8 slot right? Yep, numbers check out. 50 looks like...Cone of Cold out of an 8th level slot? Right ok good.
The problem is that mages aren't really the source of damage in 5e--or, ok fine, they're solid at AoE damage.
Like...level 20 archer, Fighter 11, Rogue (assassin) 9, Sharpshooter feat, 20 dex, let's say attacking with some kind of bow (longbow or heavy crossbow with crossbow master). If you get a surprise round, you automatically have advantage, and all your stuff crits (and crit means you get to roll dice twice, this includes superiority dice and sneak attack). Might as well action surge since we crit this round. So...6 attacks. d8 with Longbow. d8x12 = 54 damage. 5d6 sneak attack, crit increases that to 10d6 = 35 damage. You can use all 5 superiority dice if you feel like it. 5d10 which increases to 10d10 = 55 damage. Dex bonus doesn't get doubled, but that's still 6x5 30 damage. If you feel confident about hitting (you do have advantage) you can take the +10 damage from sharpshooter for another 60. Potentially 234 damage, or 174 without using sharpshooter. And now use your bonus action on something cool because you're a Rogue (probably hide so that you can sneak attack next turn). "Oh, but your damage will drop a lot without action surge and crits and superiority dice" you say? Eh...next turn, assuming you hid successfully, it's 73 without sharpshooter, 103 with sharpshooter. And of course, superiority dice and action surge come back on a short rest.
"But that's a min maxing build, and we've been comparing monoclass" you say? Ok...like...Thief Rogue with basic archery stuff. 35 damage each turn from Sneak Attack. 10 from Dex. 9 or 7 from the crossbow/shortbow. 20 potentially from Sharpshooter if you feel accurate enough (you might not--fighter gets +2 accuracy to bows). So...55 to 75. Except Thief rogues just get two full turns at the start of every combat; pretty cool ability they get at level 17. 110-150 on the first turn. And if we open up multiclassing, they don't lose much by dipping two levels of Fighter for Action Surge and +2 to bow accuracy.
OK, so maybe the problem is Rogues right? Fighter 20! Let's stick with bows. We don't have Rogue now, so there's no use worrying about using the bonus action to hide--might as well use a bonus action for an extra attack, right? So...9 attacks with a d6 weapon, or 8 attacks with a d10 weapon. Hmm...decisions. I think this depends if we are using sharpshooter (makes 9 attacks better than 8) or not (8 is better). Let's just stick with 8 for now, since I'm sure we can come up with something creative to use a bonus action on. So...84 damage without using Sharpshooter, or 164 damage with using sharpshooter. Add in some superiority dice if you feel like it (up to +39 or so). Oh yeah, and do it all again next turn, because Fighter now gets two action surges per short rest (except those superiority dice; if you burned them all in one round that is).
Anyway, point is, a 96 damage disintegrate is...well this is a level 8 spell, so it's once per day, and the second most powerful spell slot you have. Maybe it hits more? Let's see...monster manual, DC20...oh hey, I already have the page open to Ancient Brass Dragon. Mmm...well right off the bat, legendary resistances, it can just choose to make the save on disintegrate (save for 0 remember). But it only gets 3 of these per day, so maybe your party burns through them for you. OK good, your spell save DC is 19, and Dex is literally the worst stat for Ancient Brass Dragon (10 Dex; dex saving throw +6). 60% chance to hit. By comparison, our various archers are facing AC 20, with +13 to hit. 70% chance to hit, 45% going sharpshooter. Although all of them have ways of getting advantage, which means 91% chance to hit, or 69.75% chance to hit using sharpshooter.
Actually, the really insulting one is Sorcerer/Warlock multiclass (Warlock 2, Sorcerer 18, for example). Quicken Eldritch Blast. Action Eldritch Blast. 84 damage for 2 sorcery points. If you can set up Hex on an earlier turn, make that 112 damage. Take that, Disintegrate! If you're really being silly, dip Fighter too for Action Surge and 168 damage (but don't actually do that, or you won't get level 9 spells).
Not that Disintegrate is bad--it's fine, it adds a tool to the mage toolbox (without requiring a very specific Warlock/Sorcerer multiclass). But I would argue that single target damage isn't the mage's role. Even multitarget damage--Cone of Cold out of a level 8 slot hitting four people is 200 total damage (50 to each). 200 is more than the 150 that the average archer was doing, but distributing damage is generally worse than choosing targets after every attack.
So in general, most guides tell you to use magic for stuff other than pure DPS, since mages can do good work in a support role; split up/disable enemies so that they are easily cleaned up by the DPSs of the party, or buff your party.
Meteor Swarm is an exception to "mages aren't the best damage" yes. 140 damage. To all the things. At a 1 mile range. And four different 40 foot radiuses (no overlap). So...yes, that is obviously good; hit even two targets and you deal 280 in a turn. Your party's archer is like "welp! can't match that in one round, and my range is a piddly 600 feet." But it is literally once per day.
Is it better than making one character awesome across multiple turns and combats without concentration (Foresight, for instance)? Eh, debateable. Depends how many targets you hit with the AoE. Depends how many combats there are today. It's certainly competitive, but I'm not sure there's a clear winner.
Mass Heal is honestly excellent and I may be underrating it as a L9, it is "fully heal and revive party" which comes pretty damn close to "win an encounter". (Granted, not the only L9 spell which you can say that of.)
Yeah, if it was a level 8 spell I'd call it really good. Still think it's on the weak side for a level 9 spell, though.
The lower-level stuff is more interesting since the resource is less valuable. And... Life Cleric healing is kinda uber once it's maxed. Mass Cure Wounds is 36 to all allies at a L5 (+9 per spell level), which is potentially terrific, but really depends on both party size and how much the enemies distributed their damage. And for action economy, Mass Healing Word is 14/19/24 MT for L3-5 which seems potentially useful as well. But... I'm not 100% certain how much.
I've never seen a Life Cleric in action but they look extremely powerful to me on paper.
Yeah, these don't sound bad. (For comparison, Fireball on your party as a level 3 spell from an enemy would be 28, 14 if you save. Mass Healing Word comes close to countering it. For a level 5 spell, Cone of Cold is 36, save for 18. Mass Cure Wounds does fully counter that, with some healing to spare. Orrrrr you can just use your Channel Divinity (3x per short rest) to restore 100 HP...but which can't restore targets to more than half their maximum. So...similar to Lay on Hands, except instead of 100 per day, it's 300 per short rest, but can't heal targets above half HP, but can be split among multiple targets at 30 ft range.
Honestly, your healing spells are good, but by level 20 you'll probably be using your Channel Divinity in combat for your ressing purposes since you get 3/short rest, and saving your spell slots for...something? Prayer of Healing between combat to heal 25 AoE from a level 2 slot.
Life Cleric is interesting too, as I've certainly heard it argued that the best healer isn't someone who sticks with Life Cleric, but who takes Life Cleric 1 and then multiclasses into Bard or Paladin for Aura of Vitality (120 healing from a level 3 spell). If you want to get really silly, then you mix in Sorcerer for metamagic (240 healing from a level 3 spell by using Extend Spell) but you do give up level 9 spells known to swing that.
But all of that has already been discussed at length:
http://www.giantitp.com/forums/showthread.php?479038-Max-number-of-healed-HP-per-long-rest&p=20445633&viewfull=1#post20445633Including the very silly Life Cleric 1, Lore Bard 6, Sorcerer 3, Warlock 5 to get Aura of Vitaltiy castings every short rest.
That said, there's definitely an argument for straight Life Cleric, especially post 17, as they do more burst healing. Sure, the ridiculous hybrid of four classes can heal 480 per short rest (possibly burning some level 1/2 spell slots for sorcery points), but Life Cleric's Channel Divinity is 300 per short rest, and you can burst 100 of it in a single round of combat (granted, can't heal people above half health, so you still need to use spells outside of combat to get people to full).
