Like most people are doing, I'm also waiting to see the full evolution lines to really choose. But I find that, just based on the first stages, Rowlet is my favourite. I know a lot of people don't like it for having a massive ice weakness, but I think you're all just taking the easy way out. Come on you wimps, go hard or go home! ... Okay, but seriously, its super unique. Not only is it the first dual type first stage starter ignore me here, apparently I'm really stupid, it also has a completely new move, and a pretty uncommon dual type to boot. Plus, look at it it, its freaking adorable. I mean, it can completely rotate its head, that's just so awesome!
Popplio is second best for me - I kinda fell in love with some of its bonus art, it just comes across as a massive naive troll and that's pretty cool. I don't mind the design too much, but I am pretty worried about its evolution's. It has the potential to be really awesome, but due to the clown theme they could mess it up big time if they focus on that too much. Unless they make it like some uncanny valley clown... actually... water/ghost... I'd be pretty down for that. But given its upbeat seeming nature that probably won't happen. Ah well, I can dream.
Litten to me is the worst of the bunch because its so damn generic and boring. The only thing notable about it is its eyes and how tired of everyone it seems which really isn't much... There's just... nothing there really. The only thing I like is some of the Pokedex descriptions of it, which are pretty cool but other than that... I'm hoping they do something cool with it later on, otherwise I'm probably going to never bother using it.