Out of COS, its DEN or COS as my starting point and my main goal is to get to Hawaii as quickly as possible, so DEN-Hawaii, or SFO/LAX-Hawaii. Lie flats on the way are IMO not a priority I try to avoid arriving into HNL and then switching to an interisland flight. On the way home I tend to prefer red eyes and sleep, but I'm not going to fly to ORD for example, only to fly ORD-COS. Waste of time. If HNL is the required departure point back to mainland I try to spend a couple of days in Waikiki at the end of the trip. Gives the family some convenient shopping, and do another visit to Pearl Harbor. West coast red eyes are brutal IMO, so I try to avoid those. DEN isn't too bad though.

Being MEM based for OP, I would focus on ORD, DEN, LAX, SFO, and IAH. But agree it would be helpful if OP identified which island is the goal.