I'm really torn here. You can go a few different ways:
1. Rondo and Perk. This way, you preserve the two best young players on the team, with the idea that you'd build with low-cost veterans and other young players.
2. Pierce and KG. As Jon said, this is the best foundation to build a championship. You're probably going to end up with a pretty good third option picking 28th in the 1st round, and you should be able to fill out the squad to remain in championship contention.
3. Rondo and Pierce. You're keeping the best young player, and you're keeping a career Celtic who is the face of the franchise. Pierce is the youngest of the "big three", and still has enough left in the tank to win both now and a few years into the future.
3a. Same as above, but you trade Pierce for younger guys.
4. Rondo and KG. You keep a great big and a great small. Arguably, the two most important players on the Celtics.
4a. Same as above, but you trade KG for younger guys.
Topic killer post.
Hehehe.
Okay, my opinion:
KG & Rondo: ... okay, the bloody injury to KG has made me seriously doubt this one, but it's viable. Barely.
Pierce & Rondo: Would be the second best option, if you want to keep him around. However, I think his play is declining *fast*. So I don't like this one either.
Perk & Rondo: I'd go for this, if it wasn't for the fact that someone else will be able to have Gasol and Bryant.
See the thing with that last one, you'd choose youth, and sacrifice your current prowess. You'd have to make some of that up in the draft, or trade Perk + some others for say, Howard, Gasol, or whoever else is illegible, and I don't think there'll be many.
So, a very hard choice imo. Very, very hard. All of the options have their drawbacks.