Depends on where you fly to. I like Delta because I travel alot on the Shuttle. And when I'm flying elsewhere, with status, I got free upgrade most of the time. Though DL's planes are not that great (esp compared to CO), I feel that I get more bonus miles from them over the years, as opposed to CO. And Delta.com beats United.com any day.
I don't fly JetBlue anymore because their miles expired after a year.
United is not bad if you can fly their routes. Plus internationally they have advantage over DL.

I'm sure others can give you more insights.