If we're just going by starters, Totodile baaaarely beats Squirtle, who baaaaarely beats Mudkip. But if we're taking evolutions into account, it'd have to be Squirtle. I had a level 100 Blastoise my brother helped me level (I was like 8) back in Red version. I transferred it (along with my 100 Alakazam and Mewtwo, all of whom knew Mega Punch /facepalm) from every file I deleted and back onto the new one. Not to use them, but just so I'd have them with me. Eventually had to give it up as the years went by, but I don't think any pokemon I ever have could equal that Blastoise. Also come on. It's got cannons on it's back. Cannons, bro.
It's actually a tie between Squirtle and Piplup for me. I adore turtles in real life, and I might have been a pure-Water-Type-starter had I not went for Cyndaquil in Crystal. But for overall cuteness, and for his role in the DP anime, I really love Piplup. I use an Empoleon on a separate team, and he does his job well.
Piplup's design grew to me the moment I saw it. The other starters are also good, but Oshawott is probably the worst one. It's not that bad, but Samurott just looks horrible in my opinion. I dislike Samurott a lot, though probably not as much as I dislike Charizard.
Piplup is my favourite Water starter out of all of them. It's really cute to start with and it's final evolution Empoleon just looks amazing to me. I use Empoleon in one of my teams and it's not to be underestimated.