Blaziken. No need to argue on this case, Speed Boost is a very viable ability for Blaziken, since his Speed is average, with a stat of 80.
In case of Sceptile, he's already too fast with 120, so this Unburden's not too good. Apart from that, even if it gets Dragon Dance and Swords Dance, his attack is pretty low with 85. Even fully invested +2 physical attacks can't OHKO on neutral hits. Unburden is pretty situational too.
And about Swampert. M-Swampert was really viable when megas did exist in gen 7, since Swift Swim helps its bad speed, and a must in any rain teams back in gen 7. Now Megas may not return, so no Swift Swim. And about its non-mega ability..... Damp is not too good. And about the given above answer. Running Amnesia? Tell me it's a joke. Why would a sweeper run a defensive move? Although as Codeine Kiss told, its access to Flip Turn makes it better than Sceptile.
When it comes to Blaziken, he's too broken. It has access to Swords Dance, increasing his awesome attack of 120 to destroy almost anything that doesn't resist it after a +2 boost. It has been banned earlier, and even without its Mega, its still viable.