If this is meant to be only starters from Gen I to V then it'll be this:
Grass - Definitely has to be Sceptile (easily the best grass starter) Fire - This is the toughest one however i really like Charizard (then in order, Infernape, Blaziken, Typhlosion, Emboar) Water - Feraligatr it has been the only water starter to convince me to choose water instead of fire and since gen III all other water starters have been meh.
If this is including all fire, water and grass types up to gen V then it'll be:
Fire: Charizard (always will be) Water: Starmie Grass: Roserade (especially shiny version) --- Lights will guide you home
I'd choose Swampert if that didn't make me look like a "Hoenn Babbie." --- "Definition: 'Love' is making a shot to the knees of a target 120 kilometers away using an Aratech sniper rifle with a tri-light scope." -HK-47