Final Fantasy 6
-First off, the bugs that (I assume) have been fixed in re-releases, but we're talking about the original. Vanish-doom, nothing more needs to be said about that. Psycho Cyan, same. Evade literally does nothing; magic evade is used for dodging physical and magical attacks. Totally destroys item balance since you can ignore evade and stack mevade. Banon's infinite heal plus an infinite loop on the raft section lets you powerlevel by putting a stack of quarters on the A button and walking away. Relm's Sketch can literally destroy the game by glitching your inventory or deleting your saves, though it can also give you 200 of every item.
-On the topic of Sketch, character abilities. Sketch is useless, Runic is useless, Steal is useless without GameFaqs telling you what to steal from, SwrdTech is useless, Rage is OP, Morph is OP, tools are OP early game. Osmose is gamebreakingly OP. With infinite mana, nothing stops your from using your strongest abilities every turn on every enemy. Wallchange would be cool if I couldn't just use flare every turn, Ultima would be scary if I couldn't just Life 3. Etc.
-Combat as a whole is too easy to exploit. Espers giving bonus stats on level up is a horrible mechanic. The optimal way to do it is to always get +2 magic attack, so you're constantly checking how much exp you need to level, switching a +2 matk esper on, leveling, switching it off. Besides being obnoxious as hell, this makes the 9999 damage cap too easy to hit, especially since with osmose you can always use your best magic. So everything devolves to hitting as much as possible. Quick, gengi glove, gem box, etc. That's if you're not already unstoppable, which you should be. The mevade thing means your whole party can be nigh-invulnerable by their mid-30s. The Floating Continent is the game's last real challenge.
-FF6 came out a year before GameFaqs. It wouldn't be unreasonable to assume GameFaqs came out because of FF6. So much of the game is stuff no one would ever figure out on their own. Cursed->Paladin Shield, Relm's best gear, the whole Colosseum, Gau and Strago's skills, the imp set, Zone Eater, saving Shadow, etc etc. Too much of the game's content is impossible to find/understand without a guide.
-The illusion of choice is annoying when it crops up. Terra can reject joining the Returners three times, after which she's forced to join anyway. The Cid fish thing should just have ended the bad way every time. I doubt anyone chose the Golden Hairpin over Mog.
-Sprint Shoes are stupid. Normal move speed is achingly slow, but Sprint Shoes waste a relic slot and turn you into the goddamn Flash.
But now I'm nitpicking. Granted, maybe the combat seems so easy to exploit because I've played the game so many times. I first played FF6 when I was 7 years old, and I still replay it every now and then because it's just so damn good. One of my favorite games of all time.