I picked Pachirisu but switched to Cranidos as soon as I got it. :/ I wish Pachirisu's ability were more like Minun/Plusle's in the first game.
But to be honest, the game is so easy that you probably won't even NEED your first partner Pokémon. :/ By the time partner Pokémon feel necessary (or even needed in the least), you've already got a handful of them.