Maybe it's just me, but I'll take Raiden, 1942, Xevious, Giga Wing, or just about any other vertical or horizontal scrolling game where you can power up your weaponry over any other type of classic game.
Of course, I also like the whole Double Dragon, Final Fight side-scroller thing.
Hmm...I also like four player mayhem like Gauntlet or that first 4 player X-men game "Nothing can stop the Blob!".
Then again, Time Crisis and House of the Dead are a blast, especially in the hour before your movie is about to start.
Wait a sec...I also always revert to dot-munching and scenery eating games, like Pac-Man, Dig Dug and Mr. Do.
Okay...what is the best type of game ever?