I have two that I can't get enough of: Palkia and Lugia. Palkia is just a tank than won't stop wrecking you until you stand firm and fight back. I find that Spacial Rend is a better choice than Dialga's Roar of Time since you don't need to recharge.
Lugia is just the divine flappy bird. Psychic/flying combo and good moveset makes it fierce competition. I love Lugia! He's strong, and when I use him, he can take hits like a champ.