Here is my selection:
Fire: Charizard: Because it is the only starter that closely resembles a Dragon and is the most versatile out of all the fire starters.
Water: Swampert: Because it seems to be the most unique in appearance out of all the water starters (to me anyway) and is a great Choice Band user in competitive battle.
Grass: Torterra: Because it looks like a tank with a tree growing on it's back and is a surprisingly powerful attacker and wall.
Very tough decisions for me, as otherwise my choices would have been Blaziken, Empoleon and Venusaur.
These options are mostly based on design, as all starters are reasonable fighters.
Grass: Sceptile. Nostalgia also plays a factor, because he was my first Pokémon pal. Other than that, while he is a bit hard to abuse at the start, the somewhat balanced Attack and Sp. Attack help to make up for the screwed Type mess back in Gen III (where a type is either Physical OR Special). Even better, its movepool is not bad at all.
Fire: Emboar. Herp derp. Monstrous attack and health, but slow and quite fragile. Sure, he is not easy to use, but that is part of the fun, right? Flame Charge or Trick Room help with speed, for lack of better. Or some Screen Support, to keep this beast from being KOd...Attacking is easy. Just pick the right move from the vast movepool (including the Water-type, burn-inducing Scald, which could save you from some harm as well) and watch your foes burn... I mean, faint.
Water: Empoleon. I don't like Piplup and Prinplup is just slightly better. However, Empoleon I love. Its face talks serious business. And it is no mere facade. Its typing offers lots of resistances, though two very common weakness (Fighting and Ground). That is OK. Empoleon has the defenses to brace itself. This Pokémon, much like Emboar, suffers from lack of speed, but this time it has the resilience to set-up with Agility, or the infamous Substitute + Petaya Berry combo. Not fast enough? Aqua Jet to the rescue!
Cave Johnson: All right, I've been thinking. When life gives you lemons, don't make lemonade.
Cave Johnson: Make life take back the lemons back!
Cave Johnson: Make life rue the day it thought it could give Cave Johnson lemons! Do you know who I am? I'm the man who's gonna burn your house down! WITH THE LEMONS!
GLaDOS: Oh, I like this guy.
Cave Johnson: I'm gonna get my engineers to invent a combustible lemon that BURNS YOUR HOUSE DOWN!