Lamas is the #1 contender. He's beaten the top contenders and has earned the shot.
That said, the way ZUFFA decides who has next shot has more to do with name than anything and I think either Pettis or Zombie(who I think still has a bit of a layoff before his return and may be a poor choice to pit against Aldo given he's just had major knee surgery) could get the nod.
Siver hasn't fought the level of competition that Lamas has and I don't think his name value is that superior.
Too bad Mendes is going to have to wait for such a while if he were to ever get a rematch with Aldo, because I think he'll continue to dust the rest of the division, no matter who comes down from LW.
MY MMA RANKINGS
HW Velasquez Cigano Werdum Cormier Bigfoot Barnett Mir Browne Nelson
LHW Jones Machida Evans Hendo Davis Goose Teixeira Shogun Mousasi Bader
MW ASilva Belfort Weidman Jacare Okami Rockhold Kennedy Consta Bisping Boetsch
WW GSP Hendricks Condit MacDonald Diaz Maia Fitch Kampmann Ellenberger Pyle
LW Bendo Alvarez Maynard Melendez Thomson J.Miller Diaz Pettis Chandler Grant dos Anjos
FW Aldo Mendes Curran Lamas Frankie Zombie Cub Lentz Kawajiri Siver