Arcanum obviously. Basically the most broken combat engine of all time, combined with but ugly art direction and a serviceable but not particularly memorable plot. But... but... what about the worldbuilding? Sure, it's nice up until Tarant but like everything Troika it's hopelessly unfinished and after that the game falls of a cliff, which means you still have to plow your way through a great number of uninspired locations. That's right, the game isn't even good at the thing it's supposed to excel at for more than half of the game but codexers like to rate on (unfulfilled) potential instead of the actual game. But yeah, great newspaper gimmick 10/10 on the codex and nowhere else.
Plus the fact the game have such bad balancement and enemy placements that at time a player reach the black mountain mines they usually will be around level 8-12 and then theres level 25 golems all over the place.
At that point the game becomes cookie cutter and you have to do almost every avaliable side quest in a game that already have a very linear progression for the storyline (which wasnt a thing in Fallout 1 and 2)
It just ends up being super annoying, making replays a hell.
Of course, you can always sacrifice all of your party members trying to do it underleveled, but if that doesnt speak "lack of playtesting" i dont know what does