None?
^ This is what I see when I survey the league.
In fact, there seems to be a dearth of true superstars right now, guys I would put on the level of Pierce, KG, and Ray. There's LeBron, Durant, Curry, and Davis. That's it for me. And I think they're all untouchable.
Then there are a bunch of guys who are a tier below that—Chris Paul, Blake Griffin, Russell Westbrook, Kevin Love, Dwight Howard, James Harden, Carmelo Anthony, and the like. Don't get me wrong: these guys are very good and would certainly be an upgrade on anything Boston has, but they're not transcendent players, and they're not really known as being winners or having the killer instinct; in fact, some of them have MAJOR flaws—Dwight's attitude and lack of skill, Paul's tendency to not be clutch, Harden's terrible defense, Melo's volume scoring. I'm pretty sure I wouldn't complain if Boston got any of these guys (with the exception of Dwight), but I don't view any of them as enough to make the Celtics champions (unless we got three or four of them, of course, but that's not going to happen).
That said, I don't know what the ultimate solution is for this Celtics team. One of the above, surrounded by a great supporting cast including at least one rising star we draft? Maybe?