Kobe.
Pierce is going to be made to look better by the fact that he's joining a team that was already a top 4 seed in the dominant Western Conference. He has the luxury of being able to play a reduced role (possibly even as a 6th man) so he doesn't have to push himself as hard or carry as much of a load. He's in the optimal position for somebody at his point in his career - he can take it easy and make the most of his time on the court.
Kobe is coming back to a team filled with nothing but hot garbage. Without Kobe, that Lakers team should be a 14-15 seed in the West. With Kobe they might have a shot at falling in the 10-12 range.
I understand this is a Celtics page and so (by default) everybody here hates Kobe. That's fine, but people still need to show the guy some respect. Last year Kobe averaged 22/5/5 as a 36 year old coming off a major injury. In his last healthy season he averaged 27/5/6 at 34. Taht deserves respect. Over those years he's played for terrible teams, and has been forced to carry a burdern/load that no 34-36 year old should ever have to carry. Put Paul Pierce on any of Kobe's teams at the same point in their careers, see if they would have won any more games with Pierce in his place. I doubt it.
Both are great players, and like them or not both should be respected for their achievements, and for their 'leave everything on the floor' mentality.
Think about all the Lakers fans out there who hate Pierce and criticize him for being a faker, being an actor, being overrated, etc. Those Celtics fans who disrespect Kobe are no different.
Personally, I think Kobe is going to come back and average around 19/5/5 (give or take) which would be well beyond what Pierce is going to average - but Pierce is on a much, much better team and so his team is going to be far superior in the win column. Both should have nice seasons, just in different roles. Pierce will be a support player, and I think he's going to be absolutely critical to the Clipper's success - at some point, he will win them a playoff game or two with his clutch heroics. Kobe will have moments like that too, but his team will never make the playoffs so there will be not spotlight on him, and he wont get the credit.
Make no mistake, if this garbage Lakers team manages to make the playoffs (even if they get swept first round) then Kobe deserves MVP consideration. Very unlikely that happens.
But as a Celtics fan I've watched a LOT of playoff games against the Lakers. People can say all they want about Kobe being a ball hog, being a cancer, being a black hole, and all the rest of it - I don't care. There is absolutely no player on the face of this earth who scared the bejeesus out of me more than Kobe did when we were competing against the Lakers in the big-3 era. I held my breath every time he touched the ball - we had three superstars, and all future hall of famers, and we needed all three of them just to keep up with what Kobe could do on his own. That guy was an absolute monster and 100% undefendable - when he shot, you really just had to hold your breath and hope he missed...because 9 times out of 10 nothing you did on defense would affect him.
I don't like Kobe as a person any more than the next Celtics fan, but I have 100% respect for him as a basketball player.