Hmm I almost feel like for slaking it's not his fault that he's slow. It's only from his ability (he has decent speed too) which in this case is more of a disability. Almost sad when you think about it. (There should definitely be a Megaking with a better ability)
As for what I think the laziest pokemon is... honestly I'm not sure I can pick one. I hate to be "that guy" but I just kind of see their behavior as a part of what their species does to survive. Lax is like a bear who hibernates, eats a lot and sleeps a lot. I don't know if it can be called lazy because that's just what it needs to do to survive. And Slowking, slowbro, and slowpoke may be slow and appear dumb, but iirc slowpoke wasn't really lazy in the anime it was just patient. (and their was also that one that like had to go through some obstacle course to get a king's rock or something). Honestly the only pokemon that actually seems lazy is Ash's Charizard before it listened to him, but that was more standoffish.
Yeah, I probably over-analyzed this. XD