I'd probably choose [Oddish] + [Poliwag/Horsea] + [Mareep].
Oddish has a nice grass starting move [Absorb]. Does not need levelling up to learn type move.
Poliwag has [Bubble]. Does not need levelling up to learn type move.
Horsea also has [Bubble]. Does not need levelling up to learn type move.
Mareep I like Mareep. I always had one in Gold and Silver. Learns [Electric Shock] at lv.9, quite powerful for early stage in game.
I wouldn't recommend using a pokémon which is hard to raise. I don't think pokémon such as Larvitar and Trapinch are suitable for starting pokémon. More than likely their level will always be trailing, compaired to other party pokémon. That is, unless you're a super-patient-pokémon-training-machine!
But what the hell, lots of people seem to prefer a different starting pokémon, they must like a challenge in some cases.
:P