Oshawott, you know me.
It's adorable as heck, has awesome-looking evolutions, and the whole samurai theme is really cool as well. I always choose it as my partner on every main playthrough I do in a Unova game. The Froakie line comes close because I love the ninja theme the line has going for it, and who ever knew it was initially my least fave Kalos starter but became my absolute fave of the three as time went by.
Other notable mentions include Azumarill who literally got better as generations went by, from getting Huge Power to good physical STAB to eventually gaining Fairy-type and finding itself in the standard OU competitive metagame. Clauncher line due to its concept of having a claw that shoots things (and explains Clawitzer's massive claw), and also Jellicent because it's bulky and I love both of its different gender appearances alongside being one of my fave 5th Gen Water-types.