Toughest game you ever conquered

[Peter1986]

I would say "Ghosts'n Goblins", "Ninja Gaiden", "Silver Surfer" and "BattleToads" for the NES.
I have actually beaten all those games legit several times, although that obviously took quite a while.

My current challenge is to beat the Japanese version of "Ghosts'n Goblins" for the Famicom, which doesn't even have any Continues.
Yes, there is a button code for that, but a lot of players probably wouldn't find out about that.