Probably Sin & Punishment (N64, not the godly Wii version). It's still a fun game, but playing it after Star Successor did it no favors. The controls are pretty bad, and the level design and boss fights aren't nearly as good as Star Successor's.
Though if just one specific portion of a game counts, my choice would be The Answer portion of Persona 3: FES. It's a 25 hour epilogue to the main game that is almost entirely dungeon crawling, which I really don't enjoy. You also have to play it on hard mode, and the bosses tended to be really fucking annoying. For example, one boss fight had three enemies with different weaknesses, but they all had a skill that gives them a good chance of avoiding attacks they're weak to, and they also had attacks that were effective against most Personas that you would need to use. So one was weak to fire attacks, but he would also destroy any fire Personas you have, and it had Dodge Fire, so many of your attacks would miss. On one hand, the boss fights were designed so you would often have to rethink your strategy and party, but I was only interested in the story, so I didn't enjoy having to go back and level up the party members and Personas I needed.
Also, the dungeon themes were a downer. They were well done, but listening to them throughout the game was depressing.