There are a lot of old NES games that I played religiously as a kid that I still fire up and demolish from time to time, like Bionic Commando or SMB2.
The turbo tunnel in Battletoads is something I can still do easily, and find it funny how so many people reminisce about how hard it was, or how they could never beat it. That level is a complete joke compared to rat race or clinger winger, it takes me dozens of attempts to beat those two levels using an emulator and save state scumming, as a kid I remember that I actually managed to get that far without a Game Genie only once, merely because I got lucky with the level warps and farmed extra lives hitting birds on the rope descent level.