Torterra probably. He is most useful in platinum with his earthquake and wood hammer if you get him move tutored.
1. Bulbasaur: I have to admit, bulbasaur is very useful for the first 2 gyms in pokemon fire red/leaf green. I do not really find any use for him after those 2 starting gyms. Rating: 6/10
2. Chikorita: In the overview of the other Johto starters, I have to say Chikorita has to be the worst. There is no use for her really in the Johto gyms, and only really good against 2 in the Kanto region. Rating: 4/10
3. Treecko: All the Hoenn starters are all reasonably good. They are all very useful in team aqua and magma situations. He was my life saver in the champion elite 4 battle in emerald. Rating: 9/10
4. Turtwig: Imo, turtwig is the best grass starter. Not only is he useful in the first gym, but he can also be used in the 4th, 5th, 6th, 8th gym, and the elite four. He has a fantastic final evolution which has the ability to learn earthquake. He is my favorite. Rating: 10/10