For another big our main needs are:
1. Mobility - someone quick enough to defend P&R
2. Defense - ability defend the post when KG sits
3. Rebounding - self explanatory
4. Scoring - ability to score SOME points if needed
5. Potential - needs to be able to lead when KG retires
For me Smith is the top option. He's as athletic as they come, he's an elite inside/outside defender, he's an excellent rebounder, he can score when called upon, and he his potential is through the roof.
Second is Varejao - he is big, strong and tough but he's still mobile enough to defend quicker guys. He's an elite rebounder and defender, but scoring is inconsistent and his potential is limited - he's probably not going to ever be more than a role player.
Last is Jefferson. He's too slow to defend the P&R, he's a mediocre defender in general, and IMHO he's already reached his peak and will not improve further. His best skills are scoring and rebounding, but we need the other things more.