I voted Impossamole. Thirty more seconds of coding and it could have been a decent game, but alas, they snoozed for the 30 seconds it took to code the coldet and it ruined the entire game experience. Talespin gets a very, very close second for total crap playability and some of the most drab visuals ever.
So much Turrican hate! Hah, I expect it. It's one of my favorite games on the system, and it's no surprise that it owns so many players. It is a very difficult game but certainly not unplayable. You just have to remember where everything is, that's all. Maybe one day I'll do a video playthrough of the game to prove that not only is it completely playable, but also beatable. Unlike other hard games on the system, it's hard on its own merits, not because of bullshit cheap hits (like Shape Shifter) or sucky coding (like Shape Shifter, again, or The Legendary Axe).
Ballistix is certainly a pretty stupid game, but it's not so terrible that it would top my list. Yeah, I owned it back in the day and had a few laughs with it. It's really not too fun with one player but it can be pretty fun with two. Really though, I think it came out too late...three or four years prior, it probably would have been a great game.
And Yo Bro...I hate it and think it's really stupid, but at least it's technically sound. It's just boring.