I am Marriott Platinum, Starwood Platinum and Hilton Diamond (whew!). My thoughts are:

Marriott - Most consistant hotel experience. Generally poor on bonus earning opportunities and/or promotions, though those weekend certs are nice. Award availability is OK every time I've tried.. Customer service is OK -- response time is good via email but quality is not that great. Website simple and basically effective.

Starwood - Most varying hotel experience. Some are excellent, some are just poor. Generally have good promotions, though pretty dry right now. Award availability -- I have not tried so I do not know. Customer service is by far the best here, but that's primarily due to Starwood Lurker and my personal Platinum Concierge (which doesn't exist in the other programs). Website a bit too complex to use, but usually effective.

Hilton - I'm a relatively new Diamond, however I like Hilton because they are most comparable to Marriott in terms of variety and brands/hotels available, and I can usually find something in my budget for that week. I've been hopping between several of them for several months now. Quality is a little sub-par Marriott but better than Starwood. Don't know about awards here either. I got hooked on CNTU, but now don't see many bonus options. Customer service here is by far the worst of the three -- they hardly respond via email or phone. I've been having a lot of trouble with their website recently, so I've actually had to call in several reservations.

So to summarize from this top tier member, I chose the program that has the best hotel in the area, since the programs have their strengths and weaknesses. But all other things being equal, I go in this order:

Starwood
Marriott
Hilton*

With Hilton getting a nod above Marriott often because I often find better corporate rates through them than Marriott. Customer service is what sets them apart, IMHO. Of course, I travel so much my award is to stay at home or go to Vegas where award stays dont' make much sense