It's a tie between fire and electric.
I love electric because they seem to do great in battle with almost everyone. With a few exceptions. When I play the games, I always have to have an electic Pokemon... and that Pokemon is the one I use the most.
However, I love fire, too. Fire is just as powerful. And, a great deal of fire Pokemon are gorgeous. Like Ponyta, Growlithe, Torchic, Houndoom (I realize I may be the only one who thinks that...), Rapidash, Arcanine, Cyndaquil and his evolutions, Flareon and so on, so on. I also love Moltres.