I guess I will chime in, too:
1) Howard
2) Duncan
3) Stoudamire (he was a center for several years)
4) Yao
5) P. Gasol
6) Big Al
7) Lopez

Nene
9) Biedrins
10) Bynum
Next tier- Shaq, Camby, Okur, Perk, Okafor
I love Perk, but I would take all of the guys in my top 10 over him. For the current Celtics, he is a HUGE asset and I don't know how many guys play better defense than he does, but his offensive game is still very raw. There isn't enough love on here for Lopez, Nene, or Biedrins and I think too much for Shaq and Kaman.
For a future contract, I think Perk should expect about 5 years and between 35-40 million. Maybe I am underrating him a bit, but unless he is on at least a very good team, he isn't going to have the impact that he does here- where he just goes out and does his job.