I've been with the frequent flyer programs from the start. Things have changed over those 25 years and I have found that there are considerable differences in award availability at any given time between the programs. However there seems to be a bit of a rotation in who has the most availibility at any given point in time.

I've been elite with four airlines. First with US, then TWA, then AA, and now I'm gold with Delta. First and foremost in today's system, elite status produces availability. When I went from no status with DL just two years ago to gold now, I went from very difficult to redeem to pretty easy.

The OP raises the question who is best if all things were equal. That's just not possible to answer. It depends on your home airport, where you fly, what time of the year, and status. However I can rate my experience with the airlines as a non elite member for each. To do this I will exclude my recent DL gold experience and rely on 2004 and older experience on them. I will also only use recent experience on Southwest since they changed to limited availability. This ranking is for availability to go anywhere in the USA domestic routes. International is very different since Southwest would not even be included.

My list is as follows:

1) AA
2) WN
3) UA
4) DL
5) US
6) NW
7) CO*

*Limited experience.

For some reason the OP ignored Southwest. They are becoming more and more important for non stop domestic service. My home airport is TPA and WN is number 1 there in departures by a wide margin over number 2 DL.