Hmmm even after all the Turbo/PCE games I've played I'd would still have to say Bonk's Adventure, I loved that game as a kid and still do. The graphics, the music, the satisfying gameplay, the characters and bosses, it has everything a mascot driven platformer needs and then some. Its a game I still play through every now and then.
The original Bonk's Adventure still has a lot of charm that I don't think the sequels were able to match.