Bloodlines. The ending is terrible, it's janky as fuck - especially the combat, and, even with fan patches, it has a lot of technical issues. But it's the closest we've come to anyone rivaling Deus Ex. Magic stuff, flaws and all.
The first game that comes to my mind as well.
Funnily enough I initially had no real big problems with only the official patch, but on the second playthrough with fan patches and another character I had a main story stopping bug, which iirc disappeared after a reinstall, so I did at least not really lose progress.
Pity that the studio dissolved and we had no true successor since forever.
Hitman certainly also. The glitchiness of the first games was insane, but still, if you avoided those and a run worked out, it was just awesome.
An actually terrible game I still spent a relative long time in was Boiling Point Road to Hell. Several quests and the factions overall were broken when I abandoned my playthrough attempt. But I think it was the first open world fps and for some reason I loved driving the bus from one point to another. I kinda was more into the open world "racing" or Bus Simulator 2005, than the actual Far Cry before FC was GTAified like game it tried to be. It's objectively a bad game, but still I spent as much time in it as with Morrowind, which I did not finish because I got lost and bored, which is imho barely a better reason than a game being fucked up entirely.