I mostly find myself connecting in SEA, MSP, and ATL. I would rank SEA and MSP as about even in terms of friendliness. ATL has continued to go downhill in my observation with agent attitude. My last few flights through ATL have had essentially no "welcome" announcements in the gate area as well as no updates during delays - sometimes even the GA just disappears until boarding begins.

That said, I prefer ATL in terms of overall airport efficiency whenever possible.