Venusaur was no 1 the moment it got Chlorophyll, in my opinion.
What about water starters?
1. Greninja (mainly because of Protean letting it jump between types and its blazing speed allowing it to take advantage of this)
2. Swampert (Only one weakness, great at setting up rocks and phazing, and STAB Earthquake AND Earth Power.)
3. Blastoise (Mega form is crazy strong, plus it's a moderately bulky and fast spinner.)
4. Empoleon (Good typing, plus a fair support movepool)
5. Feraligatr (Though cool, he doesn't really have much but Water STAB and SD / Dragon Dance.)
6. Samurott (Same as Feraligatr, really, but slower.)
And Fire?
1. Blaziken (Mega form, or no mega form, speed boost is insane. I've seen Life Orb Speed Boost Blaziken wreck entire teams without setup.)
2. Charizard (Formerly nearly useless without support, it's now tough enough with its two Mega evolutions to warrant its place.)
3. Infernape (Nice typing, plus a blistering speed and the ability to pull teams apart with Flare Blitz. 108 speed is pretty fast, especially with 104/104 attack stats too.)
4. Emboar (While the last in a line of Fire/Fighting types, he is bulky and capable of decent offense.)
5. Delphox (Like the "fire types of old" as smogon puts it, Delphox has high Spatk and Spdef, plus fair speed. Fire/Psychic is a good typing offensively. It's frail, and doesn't have much space in its movepool, though.)
6. Typhlosion (While not inherently bad, and having pretty nice stats all around, it's just not up to par with the blistering speeds of blaziken and infernape, nor the wallbreaking capabilities of Charizard and Emboar, nor the support moves of Delphox.)