If we are counting megas, MegaBlaziken, no doubt here.. A pretty OP sweeper with a pretty OP ability, one of the main points of the anti-megaevolution crowd, he's ridiculously strong and being a Fire Type means immunity to burn so you can't even mess with his attack (which is massive).. An annoying enemy to deal with for sure..
But if we remove megas off the table I think I am more leaning on Greninja, tho' Blaziken takes a close second in my opinion, solely because of Greninja's ability to STAB any move so even if his S.Atk isn't all that high the fact he always STAB gives a massive advantage for sweeps (specially because his speed is impressive) and Mat Block is good in doubles but Blaziken still takes a very close second due to what I said above..
Tho' my favorite starter is Infernape.. I find Iron Fist quite reliable since I can have him with a decent hitting Fire and Thunder Punch (for the birdies and fishies) while having Mach Punch for finisher and Close Combat as the high damage move, his speed and attack iare also strong enough to outspeed and OHKO most things.