HP is directly affected by a character's Strength Stat; this is why raising Strength on a Mage is not a stupid idea (similarly, Magic affects MP Growth, so raising that for a Fighter serves a purpose.) IIRC, Zidane's base strength is 4th highest in the game, but due to FF9 stat nonsense, and how Freya is only marginally better, Zidane ends up being #2 in practice; Steiner should always beat him, being available early, and game best strength and all that.
Zidane does have game best spirit, though, which is worth noting when comparing to Steiner as both their best weapons use that as part of the damage equation. I think they both end up getting roughly the same damage mult from their stats, so weapons are what matter most, and Zidane has easily the strongest weapon in the game by far outside of Excalibur II. In Steiner's Credit, though, I believe Excalibur + Holy Strengthen ends up very similar to Zidane using Ultima Weapon, so they're roughly equal, with Zidane winning against higher defense (due to how the numbers work), and Steiner winning against Holy weak targets. This only matters in game, mind, cause DL Steiner should just be hurling Shock. In game, Shock's MP cost makes it actually restrictive (he can fire off like 3 shots before needing to recharge), so being able to opt for a Physical that can be twinked for 9k~ damage costing only 10~ MP (due to MP attack) over Shock's 60 or whatever is a big deal.
In any event, yeah, of the main PC cast, Steiner's probably the best, cause he's just well built for duels. Offhand, only Garnet really has an argument over him, I'd say, trading in a lot of durability for healing and (unreliable status), and a little higher speed (though the Durability should offset for all slug fest concerns.) Oh, she also can resist Holy, the one element Steiner CAN'T resist, IIRC. Steiner probably comes out ahead, since the durability edge is just that massive, with Tin Armor + Game Best HP, enough to offset all of Garnet's advantages. He even has Limit Parasitic Healing! *ignores how Blood Sword is crap*
Zidane isn't totally helpless in the DL, but he doesn't translate well despite being a good PC in game. Again, its just a case of where someone could take advantage of the game's system rather well, and become effective in game, but in the DL, most of those factors become illegal for one reason or another, and he gets slammed hard for that reason. A variety of games have a character like this, Zidane just happens to be FF9's example.
And...
Regarding Zidane vs. Luca, FF9 Blind is pretty damn good, so if you give Zidane a Thievery that's anywhere remotely close to his physical, he should be able to beat Luca with The Ogre Soul Blade + Fire Immunity <_< >_>