Well, as there are no pure grass types in Gen 3 (other than Sceptile's family, Seedot, Cacnea and Shroomish), I would recommend either Breloom (hits hard), Tropius (for flying), or Cradily (defensive).
For water dark, Sharpedo of course.
Normal: You can either go with Exploud (best choice imo), Slaking, or Vigoroth (not fully evolve but doesn't have Slaking's bad ability.
Dragon/Flying: Salamence or if you want, you can use Rayquaza post-game.
:) :) :)