I like fire pokemon, simply because they're a bit more of a challenge and usually help out later in the game. Grass is my second pick, though (go chikorita!) because it's hard to find a good grass pokemon. Water pokemon however....there's about a billion of them and when it comes to fire or grass, it's too hard to try and find one that's really good (besides Ho-oh, Moltres, Entei and Celebi)