All of them are good in winter or anytime of year. The North Platte can fish really good in Winter (no weeds). Are you looking for numbers of fish or size. For size I would fish Taylor (Very Cold), Frying Pan or the Blue and throw Mysis or midges. For numbers I would head the North Platte (Miracle Mile or Grey Reef), South Platte (Dream Stream, Cheesman) and throw small nymphs or midges, and scuds. Williams Fork tailwater can be fun or even below Wolford res. It would be hard to beat a day on the North Platte because the fish are not as fineky, they are nice sized but not too big that you have trouble landing them, and you can have 20, 30 fish days. Just don't want to get stuck in a bad wind up there. Fish them, fish them regularly and you will lay into some nice fish. The thing that really sucks is when your eyeletts on your rod freeze so try to go on those warmer days and would fish earlier or later in the winter, but fishng during mid winter can be cold and tedious. Give it a try!!
Fishinguitars - Have fished Blue quite a bit. I would say that sight fishing for the bigger fish is the way to go, but can be difficult at times. The fish in there know how to hide and they blend in really well. Lots of times they will be sitting just below the small drop structures in the bubbly white water so they can be tough to see. I think the best time to fish any tailwater is when they are realeasing water out of the dam. When I say that I mean more water than what it flows at the majority of the year. For the Blue the minimum flow has been 52 cfs and it flows at that level most of the year, with the exeption being when Dillon fills and they realease water, generally towards the end of Run off or shortly thereafter. With the higher flows it gets the fish moving around, pushes a lot of the big fish down from the dam, and they feed more aggressively. During this time there are more shrimp being pushed out of the dam and the fish seem to chow on mysis pretty good. I would walk up and down the river looking for fish. I always have a mysis on and then change up the trailer fly or vise versa, but those fish are big for one reason, their main diet is mysis. The Blue can be tough, even more so than the Taylor or Frying Pan. There are tons of big fish in there but they tend to spread out and hide during low flows. They get pounded every day by anglers. That is why I fish it during higher flows. Give it a shot!
I don't want to sound like an ad, but Karen Christopheson and Al Marlowe, from Evergreen have an inexpensive and very informative CD available that gives you 50 tailwater destinations in Colorado. I bought it and have found it very informative. Maps, when to fish, flies to use etc. in a concise fashion with great pictures. I've tried a few of the spots with better than average results.