When it comes to the first stages I love them all, but Rowlet and Litten are tied for favourites. I love Cats (even edgy ones), and I love Birbs, but my friend loves the Bird too so I'll get her pick it so she can send me a Rowlett egg, if the leaks are true, I still love the bird, and I like the Litten evo, but I LOVE the Popplio evolution and I honestly wanna use it. However if the rumors are true and we get Day/Night forms I might pick Litten if it's other form is cool enough.
TL;DR Picking Popplio if leaks are true, Litten if false.