This is really hard for me!
I love most water starters: Swampert, Empoleon (my 1st pokemon was Piplup), Samurrott, and Greninja
And most fire starters: Charizard, Typhlosion, Blaziken, Infernape, and Delphox
The grass starters that I like: Meganium, Sceptile, Torterra, and Serperior
Most starters are well done, though a few look odd, like Chesnaught, so its really hard to pick, but Water. Even though I am not a fan of Blaistoise or Feraligator, overall water has been the consistently most useful in-game type for me and they look pretty cool.