Currently several XBOX 360 and Wii games, but the ones I played the most and never completed are/were:

1. Battletoads (Original NES-could reach the level before the last one, but never get past it)

2. Punch Out/Mike Tyson's Punch Out (NES-Could reach Tyson/Mr.Dream without losing but could never beat either one)

3. Assassins Creed (X360-I can fight like it's nobody's business but I can't seem to get the pickpocket technique down)

The games that I beat the most:

1. Super Mario Bros. and Super Mario Bros. 2 (probably close to the same amount)

2. Metroid (NES)

3. Contra (NES)