Heatran is probably the best Pokemon in the game. It resists just about everything, and flamethrower and earth power are a great coverage combination. You can use Heatran.
Also after reading the Horus person's comment, I wanted to add that if you already outspeed most of your threats, any more speed is useless. On the other hand, any extra attack power is more likely to be useful. To not waste investment on speed, you can make sure you barely outspeed the threats on this list. https://www.smogon.com/ingame/bc/dp_bt_pokemon
Their base speed can be found on many sites, like Bulbapedia.
Also also defensive Pokemon tend to be less viable in 3v3. A super effective attack or a random status move, like double team or swagger, can drastically weaken a wall. Based on my experience, the best Pokemon tend to be the ones with a high attack stat and can learn either many different types of attacks or a status move that further raises the attack stat.
@guy Have you ever used Arceus in the Battle Tower? Can you show us a record?