Gotta go with DQIII on this one.
I'll give credit where credit's due and state that Famicom FFIII was probably the bigger quest, more classes/jobs, and made more clever use of its sideplots to highlight said jobs and the different ways one can influence your party. And I don't care what anyone says, I still perfer the D&D spell level idea over universal MP.
But with that said, it also has way too many speed bumps compared to DQIII (i.e. Salamander, Garuda, Doga's Manor, The Cave of the Ancients, The Cave of Darkness, and all four parts of the endgame versus the Pyramid and the Orb hunt). And then there's the redundancy between jobs where you had crap like the third crystal giving out seven classes (Dark Knight, Dragoon, Viking, Karateka, Geomancer, Conjurer, and Bard) three of which are gimmicks and four of which are fighters with gimmicks and forcing you to continue to lean on that White Wizard you made way back at the start of the game until you hit the first of the four endgame dungeons where you finally get its upgrade Shaman, along with the Black Wizard's equivalent, Warlock which is also completely redundant because you also get the Conjurer in non-shitty mode, Summoner; whereas in DQIII you get your main character who's exclusively the Hero class, and seven others including two different varieties of fighters and two different mages plus an unlockable supermage after suffering through one of the game's two gimmick classes, the other of which is actually semi-competent.
And then there's the remakes where DQIII gets better and FFIII gets worse.
Because seriously, who thought it was a good idea to turn a mostly linear class/job upgrade system into a not even half-assed specialty system? Nerfing Sage and Shaman to give White Wizard a not even a remote chance to not be underpowered crap isn't balance, its moronic. And all that making Summoner be the only non crap summon using class in the game just means that you don't need to blow an 8th level spell slot on Bahamut for your one Sage since the other one's going to be a Shaman since they don't have crap for high level spell charge totals. Meanwhile Warlock is still entirely useless since Sages can still use Flare and Shamans can still use Holy. And then there's the physical jobs but instead of it being use Fighter til Knight then Dragoon/Karateka until Dark Knights stop being useless then go with that until suddenly Ninjas now its the same until you hit Dark Knight then never stop using them as they are now better than everything until Onion Kids become viable (Ninjas hit harder than Dark Knights but are made out of paper in response and the extra 'dungeon' is nothing but Dragons which makes farming for Onion Equipment actually feasible). The only good thing to come out of that crap DS remake was making the Cave of the Ancients less of a bitch.
I honestly don't have much to say on DQIII because it may be an oldie but its still a goodie and straight up classic JRPG design. FFIII meanwhile is still as NES Hard as the other two FFs but it was finally showing some of the cleverness that let the series take the lead for the next two console generations before devolving into the corridor simulator that it is today.