Torterra is probably one of the most awesome Grass Types out there. Just watch out for Ice attacks, since most pokemon in general are faster than Torterra, he'll be helpless against types it's weak to, especially Ice. The exception might be fire, because then if he can survive one hit, he can hit it back with Earthquake.
Otherwise, Gliscor. Ground covers it's weakness to Electric and Rock, and Flying covers it's weakness to Grass. Also, it won't mess you up having two flying types, because Gliscor doesn't have any attacking flying type moves except Aerial Ace and Sky attack, which require TMs and Move Tutors, respectively. It also fears Ice, however, all ground types fear ice.
One other suggestion is Garchomp, but it is very hard to get without trading. Also very susceptible to ice types, but it can counter that with all the fire, rock, and steel types it can learn.
all of the above mentioned pokemon have a pretty solid attack stat, although Gliscor's defense is it's highest stat, it's attack is still pretty good.
