what's your favorite side scroller game

La Mulana. I haven't played a smoother side-scroller since Castlevania. And of course, them whip animations unf.

Donkey Kong Country!

Incredibly tough choice since I love side scrolling games. Top one is probably Splosionman. Also love Megaman and Sonic because they're fun classics.

Castlevania Aria of Sorrow and Dawn of Sorrow

Hmm....this is a toughie...I guess the one I may have enjoyed the most was Rayman 3 on the gba

Castlevania:SotN, Super Metroid, Mega Man Zero series

as a recent releases - castle crashers
but i really enjoyed super Nintendo's sunset riders

I don't even know, there is so many good ones out there.

Super Mario Bros 3. The absolute best side-scrolling game ever made. Seriously.

SHANK 2 :D STREET FIGHTER,CONTRA(all)(Old Nintendo version) ;)

Cave Story :D

Cave Story!

Mark of the Ninja.

If it includes platformers then I'd say Thomas Was Alone and Super Meat Boy are among my favorites along Shank and other that don't come to mind right away. If we exend the definition even more, Fortune Summoners.

Giana Sisters: Twisted dream! :D

Castlevania SoTN GG

Valkyrie Profile and the Castlevania games.

Dustforce for classic level-based challenging levels which require mastery of the mechanics.
Aquaria for exploration.
Mark of the Ninja for story and gameplay.

Retro: Duke Nukem (on DOS). Probably the first pc platformer game experience I had.

Old School Console: Super Metroid - I can play this game a thousand times over and it's still as enjoyable as the last.

Within the past few years: Super Meat Boy (This was the only game I had installed when my internet went down for a few months and I racked up a lifetime's worth of playtime).

Most recently I am hooked on Rogue Legacy. Great little game.

Super Mario World 2: Yoshi's Island, it's probably the best platformer ever

Streets of Rage 2 is way up there for me!