Isn't limited to In-Game teams, is it?
If not, the at 3rd:
Azumarill - The first one I bred had an Adamant nature, Huge Power and 31 IVs in Attack. I was pretty surprised, and even used it in a Wi-Fi battle against a friend, which almost turned into an Azumarill-Sweep... XD
At 2nd:
Vaporeon - It might not be the best, but it was the first Pokemon that I truly trained, on my Diamond. One of my friends actually doesn't like it, because it hits hard... XD
...and finally; 1st:
Glaceon - Of course, my most symbolic Pokemon. Though I haven't used Glaceon in any kind of Wi-Fi battle of PBR battle, I still love it and always try to use it in my Diamond, when I can. The fact that it has both a good attacking stat and two decent defensive stats is a bonus, but for me, never a requirement.