Simcraft will give you different weights based on how many of what stat you have. If you have a lot of one stat it'll show as less valuable and if you have a small amount of another it'll appear more valuable. As for which stat is best, pre-raid crit is the best stat for all specs. In raid gear Crit is only best for MM while BM and SV prefer multistrike. You can trust Azortharion's stat weights in his
Guide here on MMOC. He also does Icy-veins and helps to keep AMR's stat weights up to date. The AMR stat weights for are for raiding so for BM and SV they'll suggest multisrike gems and enchants. There are custom weights you can import from Azor's guide.