I'm a sucker for the Castlevania series, but that aside... hmm... a nice obscure but good side-scroller eh?
Rocket Knight Adventures, which has possibly the best premise for a game *EVER*:
You play a possum, who's a knight, with a rocket-pack, fighting to rescue the princess from another possum knight (Also possessing a rocket-pack) who's turned evil and now works for the giant evil pig kingdom who live in a steampunk noir city. You eventually invade said steampunk city except that the pig king escapes into space where he's built a giant evil pig-moon for global domination (I guess?). Luckily the princess is magic (I think?) and so using her magic you launch into space and somehow don't die, but instead go through an awesome level where you face off against meteors that launch homing rockets and floating pig heads that explode (Seriously!). Eventually you make it to the pig-moon where you face off against evil robotic clones of the pig-king (I think?) and eventually discover he's secretly only a brain being kept alive via life-support, and that all the robot clones are just a coverup to keep people from discovering he's dead (I'm just presuming at this point, it's never really made clear!). You defeat him and blow up the pig moon, only to (If you play on the harder difficulty) have to survive the pig-king-brain-thing's attacks as you plummet towards earth commander shepard style. Somehow you miraculously survive, despite not even wearing a helmet (Magic?) but the brain is burnt up in the atmosphere. Oh yeah, and at some point you fight the evil possum knight in an awesome boss battle where he's eventually blown into space (Although you survived out there without a helmet anyway, so how bad could it be?). There's also a mech-battle in there and a few bits involving robotic animals and dinosaurs...
THE END!
And it has fun gameplay too!