Squirtle, entirely because he was unlike most of Ash's other Pokemon. It's easy to see, that all of his Pokemon have different personalities, though some are more apparent than others, and Squirtle is an example of those. Mischievous from his time in the Squirtle Squad, but also determined to win his battles, and prove himself to Ash. He may not be as memorable as, say, Ash's Charizard, but he's still always been one of my favourites of the ones Ash has caught. It may be because I'm a gen I junkie, and that the first season was the only one I ever fully watched through and the first 3 movies being the ones I've watched the most, but I still think there's something about him. I also liked Infernape a lot, but that was more based on the whole journey the two of them went through to rid Chimchar of his influences from Paul, and having him master Blaze and all, but in the end Infernape turned out to be one of Ash's strongest Pokemon, and I liked that, especially because it's one of my favourites from the fourth generation. But, alas, Squirtle still beats Infernape :p