Viewtiful Joe wasn't bad.
MDK 2 I finished, but it was hard.
Zelda 2 wasn't hard at all.
The Metal Slugs can be frustrating, but beatable.
Tetris will rule your life if you let it.
Bionic Commando was VERY tough, but I beat it.
Doom I could beat with my eyes closed (I overplayed it.
Contra series was very easy except for #3. WOW! Never did beat that one.
Defender I don't remember being very hard. I'll check it out again soon (working 2600)
DMC 3 was stupid hard in the hour I spent playing it.
R-Type wasn't too bad on the Sega Master System.
Battletoads was fucking retarded hard. And, the gameplay sucked, making it an exercise in frustration.
Ghouls and Ghosts. Near impossible, but I did finally beat it....last year
Ninja Gaiden I didn't enjoy enough to TRY to beat them.