It changes constantly. For our CRJ class last year, ORD and FAT were the most junior. A lot of us went to SLC, and there were quite a few to Denver. I think for a while there was a wait for ORD, but I really have no idea what's happening now.
As an FO, there aren't many bases that you won't be able to hold in a couple of months--the exceptions are PSP, SBA, LAX, SFO, and SMF. TUS is probably next most senior, but people are holding it a month off of IOE now. I don't know where the rest currently stand, but I think people are moving pretty quicky between SLC, COS, DEN, and ORD. All this could completely change as upgrades speed up and slow down.