I actually don't know which one to pick. Tetsuja, as everyone would agree, is the coolest looking, and will probably evolve into some sort of gangster snake.
Pokabu is the cutest, and fire is my favorite type, apart from fighting at least. However, I'm worried about what the pig will evolve into. It might end up like a fatter, freaker version of Mamoswine.
And Mijamaru is probably going to be the most handy against Gym Leaders. My explanation is a bit (if not completely) irrational, but the most suitable starter seems to rotate with regions and generations. Bulbasaur was the best starter for the first three (four if you value defense over attack) gyms as well as the last one. Totodile had no basic weaknesses in any of the gyms in Johto. Hoenn's Torchic was invaluable (at least to me) for the later gyms and the Elite Four. And the cycle repeats with Turtwig in Sinnoh, being effective, through grass or ground, against all except Gardenia and Fantina. If that cycle continues, then Water types will find the Isshu Gym Leaders a breeze, so Mijamaru would be an easy starter to go with. I rest my case.
Putting all this into account, I've decided to cast my vote for Pokabu for now, but I might change my mind once I find out what its evolutions are.