It's highly debatable. I started with a Piplup, and I'm content with him now that he's an Empoleon. I've read aroud that electric type Pokemon took a hit in Diamond/Pearl, whereas fire type Pokemon are actually better. Hey, I don't know. But before you pick, keep this in mind...
Empoleon: Water/Steel. (This means he has a lot of resistances, but he isn't resistant to fire thanks to his steel attribute. Electric is his only true weakness.)
Infernape: Fire/Fighting. (It may seem like a decent combo, but he's weak to psychic, flying, and water.)
Torterra: Grass/Ground. (Not only does he suck a lot of HP, but he sucks in terms of speed. Not to mention he's got a warning tag on him that states "Do not handle in cold temperatures". Yes, I'm referring to his 4x weakness to ice...)
These are just some things to keep in mind, but just remember, none of them are truly "bad".