Starfox Zero and Sticker Star. Never bothered to finish those (not for lack of trying on Sticker Star's part). I gave up on both of the latter Paper Mario entries, though for reasons not related to quality (CS because I didn't have the bone on me and couldn't get it for the second boss, TOK because I'm _really_ bad at those kinds of puzzles the game was throwing at me)
I confess that I actually completely forgot about SF Zero. Though at least its anti-extrinsic design choices can't get you completely softlocked like Paper Mario's...