I'm not sure if a similar thread has already been made before, but I don't recall seeing one and I hope I'm not making a mistake here. So :
Grovyle and Marshtomp are kind of opposites : the first has low HP and high Speed, while the second benefits from much more HP but is cleanly slower. As for the other stats the difference is less striking. And Combusken is somewhere in between the two others and has somewhat more well-balanced stats.
Grovyle keeps its monotype while Combusken gets access to the Fighting type (and God knows how awfully useful it is) and Marshtomp acquires the Ground type (later it becomes a powerful Earthquake user).
In terms of design and originality the three are all pretty cool : Grovyle's line is the first non-quadruped Grass starter, Combusken's line is inspired by a real-life sport close to the concept of Pokemon, Marshtomp's line has a not-so-obvious origin and mudkipz are lulz.
So which one do you prefer ? Who's got the best design ? Who's the most powerful ?
As for me, I chose Combusken because I always had a kind of fidelity towards Fire starters. Plus it felt so fresh and different when compared to previous generations fire starters, I mean Combusken doesn't even have fire somewhere on its body but it still manages to be cool.