Victreebel.
An odd choice, but I used two of them in my separate playthroughs of Crystal and LeafGreen.
It's not easy to use due to Vine Whip having low PP for a mediocre attack, but Sleep Powder can sometimes turn the tide, whenever it's against the opponent or catching wild Pok?mon. Once it learns Razor Leaf and you have the TM for Sludge Bomb, it can then become a really handy Grass-type.