I think Alaska is the most generous. I flew them for the first time earlier this year: A flight from SFO-SEA, which was around 1400 miles RT. But I managed to earn over 11,000 miles for the trip by taking advantage of various bonus offers (some of them were first time flyer only). If I get an AK credit card, and the right long distance phone company, I can almost have a free trip out of this.
However, I happen to belong to several programs, so it's not worth it for me to go all out. But if you live on the US west coast, I think AK is by far the best program. Don't forget you can use/earm miles on BA, NW and, most American flights!