Probably completing A Link to the Past 100% in 3 hours 45 minutes. I know there are much faster times out there but I'm still pretty proud of that.

This may have been superseded by my effort of completing all 96 levels of Super Mario World without dying once. I actually had 91 levels done (including all Star and Special Road) but I was killed in the run up to Bowsers Castle by one of those smurfing Monty Moles. Fuming.

I also ran through all the 150cc Super Mario Kart Grand Prix (Mushroom, Flower, Star & Special) and came first in all 20 races.