I highly recommend the Poly. I have stayed there all seven times I have visited WDW and every single visit is am awesome experience. I cannot say enough good things about the resort. I have never stayed at the other Monorail resorts, although I have eaten at them. I had a friend who stayed at the Contemp back in '93 and I didn't like her room at all compared to the one I was staying at in the Poly. Here's my take on all three.
GF
Very elegant. Stuffy and kind of plain if you don't like overly fancy resorts. The pool area is very nice and interesting, so if you have kids that is a plus. The theming is good and landscaping is very nice as well. The restaurants are very nice, but I actually don't like them as much as I like the other resort's premiere dining experiences. It is light and airy like the Poly, but it has an almost snooty feel to it. Not my cup o' tea at all, though.
Contemporary
Very modern, urban and not very unique in terms of theming. It is nice and the California Grill is hands down the best restaurant in WDW. I even prefer it any day over the GF's Victoria & Albert's. The landscaping at this hotel is also not on par with the other two hotels in my opinion. I think it has a rather cold and lonely feel to it.
Poly
I might be biased but I believe it has the best atmosphere overall. The views are better here than at the other two. The food is very good and the entertainment and overall ambience can't be beat. The pool area is the best one of the three Monorail resorts, too. It has the best transportation out of all three, as well. You can take the monorail, boat launches or ferries to the MK and the other resorts. And the TTC is literally within 5 minutes away. My room this last year I went was only about a 2 minute walk away. I think the cast members at the Poly are also much more friendlier and more talkative. You truly feel as though you are on vacation when you stay at the Poly. Like I said, I'm biased, but I think it's WDW's premiere resort.