Hmm...excellent topic, ryan; I've considered it, and come to the following conclusion:
Beedrill-Overshadowed by Butterfree. I used it for the first half of Leafgreen and it definitely earned its keep. It's also one of my favorite pokemon stylistically, and has always been the "nasty surprise in a tree" pokemon of choice in the anime.
Venomoth-In honor of my shining venomoth on leafgreen. I have always liked that it can use psychic attacks as well.
Shuckle-My favorite pokemon, and the only pokemon I've selectively bred and EV trained to level 100. Also in my favorite episode of the anime.
Forretress-One of the pokemon I was most fascinated with when GS was new; though it proved to be tricky in battle I've always loved the way it looks; and that the anime was merciful enough to give Brock one, a surprising move in a time where they only allowed popular pokemon.
Heracross-Despite Ash owning one, I always enjoyed the thrill of the hunt of searching for one in GS. I''m sure I had to headbutt a thousand trees before I found one, and even then, it ran almost every time I attempted to catch one. It was also the first bug to know Megahorn; and its fighting skills were always invaluable to me whenever I used it.
Shedinja-One of the most creative pokemon; between the way it's acquired, its HP, and Wonder Guard, it's without equal in its usefulness. I'm also a fan of the way it looks.