Final Fantasy Tactics Advances and Final Fantasy Tactics A2. It's always the stupid fucking "learn skills from weapons/armor" system that I hated in Final Fantasy IX; in FFTA, the RNG would screw me out of the item you need to learn Steal Weapon which is turn absolutely vital to not missing out on tons of other good shit. I don't remember precisely why I could never finish FFTA2, but I think that again, the RNG never gave me the loot I needed to make the items that had the skills I wanted. The last Final Fantasy Tactics game was Final Fantasy Tactics S, a shitty mobile game that flopped even in Japan, so I think the series is buried for good now. A pity, but modern Square-Enix has lost its way and will probably never find it again, so a new Tactics game would just be more mobile dogshit anyway.
Echoing many previous respondents, Final Fantasy XIII. I pushed myself too hard to get past the opening 20+ hours of tutorial and completely burned out on the game (which I didn't like much to begin with); as soon as I could finally switch party members, I was overwhelmed, turned the game off, and never turned it back on again. Man, that game was just complete ass. I hated it worse than II, and that game was so broken that I will never understand why it didn't just kill the series right there in the Famicom era. I mean, I'm glad it didn't, because we got to enjoy FF IV-XII and Tactics, but it was just bug-ridden garbage.
Incidentally, I am like 95% sure that the entire reason FFXIII-2 was even made was to salvage what they could from literally millions of dollars worth of wasted visual assets made on the Crystal Tools engine. An interview with the FFXIII dev team a while back confirmed that the production was a total clusterfuck with none of the sub-teams talking to each other and they made twice as much stuff as was eventually used in the final release version. Furthermore, the catastrophic failure of FFXIV 1.0 and redoing it practically from scratch with a new engine meant that it, too, had a huge amount of unused very expensive visual assets gathering dust, and I seem to recall some hackers confirming that some unused areas in FFXIV made their way into FFXIII-2. I'm also pretty sure that this is why they decided on the time travel theme too, as this was just an excuse to have very different areas that would never fit together in the same game. The poor reception of FFXIII would have never justified a sequel on its own merits, it was something much closer in spirit to releasing an untested, bug-ridden, half-finished game because the company went bankrupt, nothing is ready to release, and the creditors want something back. While SE never actually went bankrupt, FFXIV flopping had the company in some dire straits for a while.