Hmm. Best? or Easiest? I'd say neither, but then I dont think any of them are 'best' or 'easy'
I'm (pre-end beta) a high gold level player. Mainly protoss, sometimes zerg if i want to have some fun. Having moved through levels below gold i have to say that I've never seen terran get such an easy win in those leagues.
Maybe if you're going from diamond level timing then yes you'll get the easy win. But the same goes for
any race. If i went up against Nazgul he could probably just 2 gate me and win because his timing and macro would just be so much better than mine. Doesnt matter that he's protoss though.
At higher levels, I suppose I can't really comment because i'm not...yknow...good. But it's worth remembering that Terrans arent going and winning all of the big tourneys. Seems to imply that there's nothing too overpowered about them.
I dont know what it's like on the platinum/diamond ladder though, so you could just say i'm wrong and i'd accept that
It's also worth considering maps. Some would say that zerg is overpowered on Metalopolis, similarly terran on incineration zone. Dunno...you're right that the build possibilities are very diverse in terms of quickly countering opponents though. Probably why TLO's taken so much interest in them recently..
EDIT: Ninja'd