It's a tough call, but I think I would say fighter.
Two 1d10+1 healing surges per short rest plus theoretical 19 AC (chain, shield, defensive fighting style) can be massive. With a 14 Con and two short rests, that can get us 12+(1d10+2) (regular HD spending during rests) + 4x(1d10+1) = 45.5 hp.
Paladin with 13+ str, 14 Con, and 16 Cha edges them out by a point with 46.5, but no fighting style until level 2 keeps their potential AC down a point. That, plus it's significantly more likely that you won't have both a 16 Cha and a 14 Con, since you probably want to get that Str up at 16-17, and it's a lot easier to get that to work with at least one of those other two a bit lower.
A raging barbarian can double their hp, which are 14+(1d12+2) =22.5 (so doubles to 45 even), so almost the same, but their AC is very likely to be lower (whether using unarmored defense or armor) and any hit before they rage eats into it.
Cleric that heals themself gets close, but not there, plus similar to paladin has a harder time getting a 14 con and a decent wisdom along with the Str necessary for optimal level 1 AC (I am treating wearing armor without the requisite strength as violating 'decent offensive output,' as never getting to the fight is generally a way not to contribute to it).