Gunstar Heroes for the Genesis is a must, truly one of the greatest, most engaging shooters out there. It was followed up with a sequel, Gunstar Super Heroes for the Gameboy Advance. It has a greater supports on melee attacks and sacrifices a small amount of quality (the game is still fantastic) for a much greater amount of variety. It also has different storylines depending on the difficulty, lending the game a great amount of replay value, though I sorely miss the original's weapon system and co-op.
Also don't miss Alien Soldier, GH's spiritual sequel which has a far greater focus on a ridiculous amount of challenging, creative boss battles.
Speaking of Treasure, Dynamite Headdy is another Genesis classic. An astoundingly creative and sadistic game about puppets.
The Klonoa series offers a more cerebral approach to progression and each one is a handcrafted masterpiece of 2D (or 2.5D) gameplay.
Gish is an overlooked, very unique and very entertaining platformer in which you play as a ball of tar. It's physics based, which means it can be likened to a slower version of Sonic, but that wouldn't do justice to how the game world affects our hero.
Just played the Noitu Love 2 Demo, and just from the 1.5 levels I was able to play, I can tell it's something special. Combat is handled in a way far different from the standard fare with it's focus squarely on the player character's mobility and melee potential (though there are ranged attacks).
Viewtiful Joe is more on the beat-em-up side, but it's one of the most stylish, well-made platformers for the post-PS1 generation. It would be standard if the gamer didn't give you control over certain editing effects (the game takes place inside a movie) that give Joe certain powers. I haven't played the others, but I heard the gameplay is more or less identical.
And, of course, there is the Contra series. Though the series has had a few forgettable 3D missteps, when it works the games offer a highly traditional but blisteringly fast brand of gameplay. I'd suggest the following:
Contra (NES)
Super C (NES)
Contra 3 (SNES)
Contra: Hard Corps (GEN)
Contra: Shattered Soldier (PS2, differs from the originals in many ways, with a focus on boss battles, a more rigid weapon system and better replayability)
Contra 4 (NDS)
If you have a PSP, check out LocoRoco 2. Unlike the first, it has a lot of depth and substance in addition to its undeniably charming style.
EDIT: God, how could I forget N? There is a free version for the PC (doesn't scroll) which is still a fantastic platformer, but it's quality is eclipsed by it's sequel, N+ which has three entirely different versions on the PSP, Xbox Live Arcade and DS. There are no weapons, bosses or plots (save for a tongue-in-cheek intro), just room after stylized room of creative, hardcore platforming with a unique and complex physics system and an agile main character.
PC
Mac
Linux
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Director of the Department of Orderly Disruptions
[Last edited by Snacko at 03-08-2009 05:59 PM]