Easily:
Torchic(HA, simple)
Froakie(As same, HA)
Cyndaquil(Once it get eruption in full evolution, it's really strong with the speed and HP),
Mid easy:
Snivy(Get on the high level for the leaf storm, plus contracy damn)
Chimchar(HA, but can get easily defeated)
Normal: Piplup(I don't think it will be that hard, got some decent moves and steel type tho)
All the kanto starter pokemon(They are decent enough for being normal),
Toditile(HA tho)
Mudkip(Get earthquake and hammar arm in full evolution, strong moves)
Fennekin( It's decent enough),
Chespin( Despite the flying weakness in full evolution, it have really great moves enough for me being normal when it's evolve).
Hard: Oshawott( Doesn't have that good movepool enough)
Tepig(it's not that strong compared to the others imo)
Cikirotha(Isn't really strong in near in reborn levels, maybe the weakest of all starter).
Starter i dunno set the starter: Turtwig.
Just something. Don't use starter as Piplup, Mudkip, Tepig, Chikirota and chespin. You can catch them in the story later of the game.