Grass types are one of my favourite types because of the fact that they can fit so many roles at once. My choice of starter in Sinnoh is usually Turtwig, which thus leads to me having that as my Grass-type. If not, then Roserade usually ends up being my choice, even if I refuse to evolve it until the begining of the league.
I believe that I used Leafeon at one point, but I much prefer to use Pokemon with a dual typing, meang that I'd much prefer to use those ones. As such, I usually end up with any of the below; Torterra, Roserade, Wormadam, Abomasnow and Tropius. Unfortunately, I find catching Burmy - especially the females - a real pain, and Abomasnow are caught too late in the game, so I usually end up falling for Torterra, Roserade or Tropius. I have traded in a Grass-type once or twice though, Exeggutor and Breloom being the latest. I sure do love the Grass/Psychic and Grass/Fighting typing, and then Breloom's Spore move just makes things as fit as a fiddle.