I think all three of them are beautiful. It's about time for something to change the Kanto Pokemon up about it, and I think they are so aesthetically great, and will become pretty awesome Pokemon to use in battle!
As for which one I will choose, the choice is hard. I already know for a fact that Chespin will be my starter of choice, unless its evolutions are god-awful, then I will probably choose Froakie. However, assuming that Chespin will be my choice, I am stuck between Blastoise and Charizard. So far, there are not many water types introduced that I particularly like. Clauncher is swell, but a bit boring to me, so Mega Blastoise would be so awesome to have! I've gotta hand it to Mega Charizard though. For a long time I just didn't like Charizard a whole lot. I didn't hate it, but I just wasn't into it. With its evolution though I really really like it! My dilemma with it though is that I think Houndoom will be in-game and c'mon that's my favorite Pokemon ever. If not Houndoom, I know we get Torchic and wow I really like Blaziken and its Mega evolution. The choice is just too hard :c
I'm soo so happy that all this happened though! Game Freak has scored again with these Mega evo's, first with Mega Absol and now with these. (I'm not saying the other Mega evo's aren't any good, but I mean come on... Absol looks like every fangirl's fan-art since forever)