Although I do it casually, I like to speedrun Sonic 2 from the Genesis/Mega Drive. Some of my best times on individual acts have either tied the records or been one or two seconds off, and if I were to combine all my best times, I'd be somewhere right around 50th in the world.
If I had the time and patience, I'm sure I could greatly improve on that, but with all of the warps and single frame tricks that some people use, I'm happy with just being pretty good rather than perfect.