Hey, you evolved your Grotle. Nice!
You should definately wait to hear the opinions of some of the other TPMers, but, that's a tough decision. Both Drapion and Skuntank are very popular pokemon. Skuntank has great HP and solid attack and speed. Drapion is faster, slightly less powerful, but has great defense. Your team already has a special tank, and both Gastrodon and Torterra have pretty good defense stats, so, I'd suggest a Skuntank, who can sweep and last. Everyones using a friggin' Drapion these days. Of course, Gastrodon is also an HP tank. If you're willing to deal with it's lameness, Heracross is also a suitable addition to your team.