GSP's loss to Serra was a big mistake, that's really what seperates him from Anderson in the best ever discussion, not that either should really be mentioned at this point.
GSP's beaten Sherk(a LW), Trigg, Hughes, Koscheck, Karo and then the draw with Penn. Those are his top victories, but he also lost to Serra recently who is not that great and Hughes not that long ago, but he redeemed himself and then some for that loss. Other decent wins include Mayhem and maybe Spratt or Hieron. I have a rating system that is too complex for you to understand, only Mullet can comprehend the complexities involved. Anyways, I give GSP's career a 8.6.
Anderson has beaten Hendo, Franklin x2, Lutter, Marquardt and Horn. Those are his top victories, he has lost to Takase and Chonan, but that was 4-5 years ago, and Azeredo in his MMA debut. Other decent wins include Riviera, Murray, Steibling, Stout, Rivera, Newton, and Sakurai(he should possibly be in the top wins list, but is much smaller even though they fought at the same weight, 167). The depth of his secondary wins is what seperates them the most and GSP's recent loss to Serra. Anderson has a rating of 9.0.
I don't know as far as clearing out divisions goes. I don't think any of the guys you mentioned will give Anderson much trouble, and I doubt Lindland or Lawler are coming to the UFC. I don't see anyone giving GSP much trouble either though. I'd love to see them meet at a catch weight.
Right now I'd give Anderson the edge over GSP and neither are that close to Fedor, but another weak year out of Fedor and an incredible year out of Anderson could bring him close.
Fedor is better at just about everything than GSP, P4P, not just mentally. GSP has better shots and sprawls and that's it. Fedor has better stand up, better throws(although that was a nice throw on Hughes) better submission, better heart. I don't see Arona/Fedor being equivalent to GSP/Penn, because their was no striking on the ground allowed, which is Fedor's strongpoint given his hand is healthy.