I vote Arceus, personally. I mean, come on guys. He's a frickin' god pokemon. Who can beat that (with the exception of uberchu of course)? Yeah, Darkrai and Shaymin are great, and so are the Dragon Trio, but my easy second is one most haven't made much note of.
The MIRAGE Trio. Why? Because they've got all your basics. Uxie, which is pretty awesome as it can put up both walls and has huge defense anyway. Azelf, a beast of an attacker with great stats in both. Mespirit, which is little more than a healer, but I still like it.
After that, I like Heatran purely because its pretty awesome. With fair typing, they have a great variety of special attack bonuses. Heatran simply owns if you use him right.
In short, the god pokemon gets my vote, but the mirages and the volcano come in really close.