A big +1 for CO CLE crews. They're some of the finest in the business, (sorry) at least as far as US-based UA/CO crews go. There's some great FAs based out of IAH as well. I've had mixed experiences with CO's EWR based crews mainly on SFO-EWR flights over the last few years. I've definitely seen that "New York attitude" which was quite shocking and offensive the first time I flew that route. Domestic UA crews generally don't have that attitude, but most of them are pretty indifferent. I couldn't name a specific US hub where I've had better FAs than another on UA.

Fortunately, nowadays I mainly fly UA to/in Asia and I get a lot of NRT and HKG crews which are really great. Perhaps older than the average Asian airline crew, but they're always helpful, patient and attentive. I actually choose to fly UA in Asia even if there are other options available.