D&D 5e
Yes, yes I'll get back to politics at some point.
But...while I've been sick, I've spent a good deal of time curled up with the nearest book...which happened to be the 5e PHB.
It got me thinking about how...some classes seem to just stop getting stuff at higher levels. Like...Circle of the Land druid--they get extra spells known up to level 9...and then nothing more through level 20. They get new wild shape forms up through level 8...and then nothing new through 20. They get a decent level 10 ability, a very mediocre level 14 ability (the highest beast has CR8, so it's unlikely to come up in a level-appropriate encounter). Granted, druids share abilities at 18 and 20...buuut they're both wild shape abilities (which Moon Druid uses much, much better).
Alright, so characters at level 20 not multiclassed. I'm going to focus on full casters to start...
Wizard
Is a level 20 Wizard good? Well yes, it's D&D, don't act too surprised. Some of the paths are really good on top of that. Like...Illusion gets Illusory Reality, letting you turn something from an illusion spell into a real object. So...cast a level 1 illusion of a stone wall, and then make it real. Congrats, you just used a level 1 spell to do what is normally done with a level 5 slot (Wall of Stone is level 5, and a pretty good spell). And regardless of path, you have lots of versatility, arguably the best spell list, etc. So like yes, Wizard at level 20 is pretty good.
Warlock
The problem with Warlock at 20 is that the Mystic Arcanum feature (what they get for level 6-9 spell slots) is really trashy compared to level 6-9 spell slots. Literally one spell known for each level. No ability to use higher level spell slots. No second spell for the 6/7 slot. The choices are about a third of the choices Wizard gets for 6/7/8/9 spells (with two spells Wizard doesn't get; Conjure Fey, which is fine, and Glibness, which is trash). Fortunately there's pretty strong options at each level...although being locked into one spell forever still hurts. (Like...Demiplane is a great spell....for Wizard. For Warlock it means they never use their level 8 spell for anything else ever. Same for something like True Seeing).
But OK, that's really not the strength of warlock. Let's focus more on pact magic. 4 level 5 spell slots. That's awesome, right? Slam out 4 level 5 spells win a combat, take a short rest, and get all your slots back! Well...it's not as good as it sounds. Warlock's level 5 PHB spells are...Contact Other Plane (talk to deities), Dream (talk to people in their sleep), scrying (spy on people), and Hold Monster (aha, a spell that has some value in combat. Although it's a single target all-or-nothing status that offers the target a saving throw every turn, takes concentration, and lasts a maximum of one minute). Through the various pacts, there's also Dominate Person (An arguably better status than Hold Monster, but only works on humanoids) Telekinesis (restrained is weaker than paralyzed or charmed, but the spell keeps going if they break out, and you can drop them off cliffs), Flamestrike (fine, but comes with the Fiend path that also gets Fireball, which is more damage when cast at level 5), Hallow (oh wow, this looks like a pretty awesome buff or debuff. No concentration. Mass protection from elements or silence or fear or....wait, casting time 24 hours?
? WTF? Nevermind, this sucks), Seeming (mass disguise self? Uh, ok).
So...I guess it's more like casting a level 4 spell at 5th slot? Well...again, only four level 4 spells in the Warlock spell list. There's Banishment, which is pretty good (Two targets, get to save once and then they're gone for the duration of the spell--requires concentration). Dimension Door is nice to have. Hallucinatory Terrain is whatever; as much as I hyped up illusion Wizards above...even they don't get too excited for this one. Blight is...8d8 singletarget damage (for the two paths that don't have fireball, this is good. Compared to Fireball...cast out of a level 5 slot, Blight is 9d8 averaging 40.5, and Fireball is 10d6 averaging 35. But you know, multitarget and all that). The various patrons give some good options at this level (Greater Invisibility and Evard's Black Tentacles jump to mind. Although...as good as those two spells are, they gain little from being cast out of a 5th level slot instead of a 4th level slot).
So...third level slot? Now we're talking, with 12 (!!) spells in the default list (and more in other lists). Although a lot of these also don't gain anything from being cast at 5th level (looking at Hypnotic Pattern, Fear, Gasseous Form, Hunger of Hadar, Major Image, Remove Curse, Tongues). So...stuff that does scale up is Fly, Counterspell, Dispell Magic, Vampiric Touch. None of these are super amazing if cast out of a 5th level slot compared to a 3rd. Regardless, good variety, decent amount of power in this list, even if not much scaling.
So...actual fifth level spells are concentration single target status with per-round saves, almost exclusively. Buffing, AoE, and battlefield control start at 4th level. 3rd brings lots of stuff. Hunger of Hadar is a unique warlock spell at 3rd level, with some pretty interesting properties (no save blind while in the area, no save damage--albeit not much at 2d6, difficult terrain, 20ft). Not sure if that's better or worse than Evard's Black Tentacles, which also makes difficult terrain, also 20ft, and restrains targets and deals more damage on a failed saving throw. Mmm...actually, due to the way advantage/disadvantage work, blindness doesn't mean a whole lot for Hunger of Hadar, because anyone attacking into the area can't see their opponent either, so the advantage/disadvantage cancels out on both sides. Restrained is restrained, though.
Oh, quick comparison, Eldritch Blast deals 42 average damage at this level when you hit
So...ok, all hail Cthulu, pick the Great Old One, praise the black tentacles. Has synergy with the shoving from Eldritch Blast pushing opponents into the tentacles. Or maybe pick Fiend if you feel like casting more than one non-Eldritch Blast per combat, cause literally all these spells require concentration (as do many of the good Mystic Arcanum) and Fireball does not. Part of the draw of Warlock is having four pact magic spell slots to burn every combat, and then getting them back with a short rest. Then again, if you want to burn spell slots in a combat, Dispel Magic, Counterspell, and Dimension Door would be happy to eat those spell slots, and are probably more relevant at 20th level combat than a fireball cast out of a 5th level slot.
So...ok, 4 3rd-5th level spells, depending on what kind of effect you're looking for. (We'll average this as 4 4th level spells per short rest). This isn't bad for sustain. A typical Wizard would have 9 spells in this level range, and be able to gain 3 more on a short rest. 12 spells in that range. (Plus whatever the wizard can do with 4 level 1 spells, 3 level 2 spells, 1 level 6 and 1 level 7 spell...that's probably more in the 20 range). Of course, level 20 Warlocks have a capstone where they get one short rest for free, which brings their starting value to 8, so...3 short rests to match the wizard, 4 short rests to out-sustain the wizard? Sounds...ok. I don't think you're really going to out-sustain Wizards for the most part, but with enough short rests you keep up fine.
So...Mystic Arcanum are underwhelming. Pact Magic is...able to keep up given the right playstyle, but can fall behind without short rests. What are the big advantages?
Well...Eldritch blast is good. The best Wizard cantrip deals 22 to its 42. Wizard could take Spell Mastery with Scorching Ray for free Scorching Rays...but that only deals 21 damage.
The other big selling point is Invocations. Let's see...
I see a lot of hype about Darkness/Devil's Sight "cheese". That said, being unable to see doesn't do as much as I thought it would. Makes some spells that require vision uncastable. Gives disadvantage on their attack rolls (if you can see them, which you can). Gives you advantage on attack rolls against them. These are...all nifty, sure, but you could also just take the Minor Illusion cantrip, create an illusory cardboard box over yourself (with small peepholes so that you can see out, and a small hole at arm level to attack out of) and...get the same three benefits (they can't see you, so no sight spells, you have advantage, they have disadvantage). (You can't move this cardboard box illusion, and an enemy could spend their action doing an investigation check analyzing your illusion to see through it. But eh, cantrip instead of an invocation, a spell slot, and concentration). That said, we're talking level 20, and I expect a good number of enemies would be packing some form of Truesight by now, so neither Darkness/Devil's Sight nor Minor Illusion should be pulling that much weight.
That said, some of the others are pretty good.
Witch's Sight. Not quite Truesight, but really good.
Book of Ancient Secrets for all the rituals (literally). Most are divination spells, but Leomund's Tiny Hut helps you get short rests, and Find Familiar is a good spell.
Visions of Distant Realms. Arcane Eye at will, always know what's ahead.
Ascendant Step. Levitate self at will.
One with Shadows. Invisibility at will that doesn't use concentration.
Eldritch Sight. Detect Magic at will. (Although detect magic is also a ritual, so maybe skippable. Lots of other at will options like Silent Image. Or whispers of the grave to interrogate people you killed).
(And you probably want a couple of your 8 invocations going into Eldritch Blast).
All in all it seems like Warlock's role (at 20) is heavily divination/scouting out of combat focused. Rituals get you a large number of Divination spells. The 5th level pact magic options were 3/4 Divination spells too. But their invocations in particular--many of the good ones are just big upgrades to what they can see or learn, and stuff that is on all the time like With's Sight (helps avoid surprises for the party).
So...mmm, that's a niche, at least. (And one that...a Wizard would need to cast like...True Seeing to achieve, a level 6 spell that lasts for one hour. So...Warlock definitely does this better). Although the irony, of course, is that the typical Warlock won't have much wisdom to use on perception....