Cormier would destroy Lesnar, and Barnett probably would as well. Barnett matches up well with any HW in the UFC. Against Velasquez or JDS, it would be a close match up, but Barnett's experience would likely be enough to carry the day.
Cormier is still a developing up and coming fighter. Kind of like JDS and Cain. Nobody knows who'd win, but the 4 of them would all match up very well together. As for Overeem, at this moment I'd probably favor him over any one of these guys.