I'd go for piplup,
My reasons are kinda personal and game related. Starting with Turtwig, i just don't like Torterra... For Torchic, a pokemon i love, it's more of a game related choice.
There are plenty of opportunities to catch a good and strong fire type (Arcanine, Rapidash, Magmar, Ninetails), while a good water type is harder to find, even more in the beginning.
I have played Reborn only once untill the end of ep. 9 and only know of some events, so i am definiately not the Reborn master you might need, but my opinion wont change.