IMHO, a lot of it depends on you want to connect in the states. I've never been too partial to DL's hubs in ATL, and I'm definitely not a big fan of CO's EWR hub (which sounds like the best bet for yoru travel to ORD). I would recommend NW because of DTW, which IMHO is a great hub and a quick hop from ORD. You might also consider CO, which can route you down south from ORD, picking up the extra miles.