Adventures of Dino Riki [NES]
Why this of all things? It's not because it's a necessarily good game to most, it's just that it could have been a decent game if some aspects of it were really tightened up. It's nothing more than your standard vertical SHMUP with some nasty jumping elements in there that are thrown off greatly by poor controls, but it was an interesting concept overall.
Then again, the game was plagued by stupid design functions. As much as I like Dino Riki and, say, King's Knight on the NES I'm pretty glad that a lot of SHMUP designers (save for R-Type, which I love anyway) moved far, far away from the "if you didn't memorize this moment you're going to die" situations. There's nothing worse about playing when you notice that unless you're on the other side of the screen, prepped to jump, and have a particularly powerful weapon on hand you're just going to die.
And I also throw in some love for Castlevania: Lament of Innocence, Rygar (PS2), Shikigami no Shiro 1 and 2 (PS2, Mobile Light Force 2 and Castle Shikigami 2 respectively), and...yeah, let's toss Star Ocean 3 in there.