Neither Cain or carwin can be considered the best heavyweight in the world yet.. They have like one fight each against big names oponents.
Other than that its: Can1, can2 and the tomato can.
According to the UFC, Brock Lessnar is the best fighter in the world.. And so was Silvia.. Brock, Cain and Carwin has nothing on their record to even slightly suggest that they are the best. It seems these days it just takes fighting 2-3 times in the UFC for the UFC fanboys to jump around kissing and screaming "The best in the world", pretty much what happened in Brock Lessnars case.
None in the UFC Heavyweight division has as impressive record as Fedor, being undefeated, fighting so many top names in their prime. Brock won't be able to keep his belt for long, cus the reality is he isn't the best, and never was, he is a top HW 10 fighter, Cain/Carwing probably top 15. Fedor is still dominating. Fedor isn't ducking.. And still fighting top 5 guys. While Cain, Carwin for the most part isn't.