i am sure that most of them work just wanted to hear first hand opinion what good. also any ideas on fabric wieght (warmer/not so warm)
I have Capilene 2, Capilene 4 and a Patagonia Merino 3, all 1/4 zip T-necks. I also have 2 Patagonia R1 1/4 zip T-necks that I sometimes use as a base layer and sometimes as a mid layer. I like them all and what I wear depends on the weather forecast for the day and I always have my down sweater with me in case it turns out colder than expected. I don't think there is a lot of difference in the warmth of any of the base layers so I wouldn't worry about buying a heavier weight base layer for CO. In CO in January you will need a base layer, down sweater and a good shell. You get the warmth from the down sweater and shell(blocking the wind). As Whiteroom stated the base layer is to move moisture away from your skin. I really like Patagonia's merino blend, not inexpensive but well made and very comfortable. You can use either the Merino 2 or 3 in the Sierras and in CO. I typically wear only Capilene 2 or Merino 2 for bottoms under shell pants and my legs are fine.
For baselayers, I own a Patagonia Capilene 1 stretch silkweight top and bottom, a Patagonia Capilene 2 1/2-zip top, a Patagonia Merino 2 1/2-zip top, an old Marker Powerstretch 1/2-zip top, and a REI Heavyweight baselayer bottom (very thin fleece).
I used to use wool tops and have now moved to synthetic tops most of the time. For bottoms, synthetics is a no-brainer for me. Lower bodies don't stink, and aren't as susceptable to feeling cold.
Merino wool tops are nice for the stink factor and are a bit warmer when wet, since the water is trapped inside the fibers and won't create as much evaporative cooling (dries slower as a result). However, they wear through pretty quickly (little wear holes). My wool layers lasted only a season or two when I used to use them heavily. I now save wool for human-powered overnight trips, when the stink and warm-when-wet factor can be a pretty big advantage. Patagonia's lifetime warranty/repair program makes it a no-brainer to buy from them. They also use a blend, which makes them a bit more durable.
I prefer Capilene (synthetic) tops for day trips and skiing. No need to worry about stink since you can just do a quick handwash every night - toss it in the bathroom sink and swish/scrunch it a few times with some soap. It's a lot more durable, and since my baselayers get heavy use, it's nice not having to take a merino wool top into Patagonia every season or two.
I really like the new Capilene 1 stretch material for tops and bottoms - it feels great agains the skin and the stretch hugs your body really well and gives it a smooth finish, both of which is really nice for layering. It also spreads out moisture very quick. Bonus points for the girlfriend eyeing me more often in the tight fitting shirt.
I use a powerstretch 1/2-zip as a top baselayer on colder days - the brushed fleece interior feels great against the skin and really wicks moisture to the outer layer of the fabric quickly. The different fabrics on the inside and outside of the powerstretch fabric means the moisture never stays next to your skin - it's quickly moved to the stretch fabric on the exterior. The smooth exterior also makes layering a bit easier - not as good as Capilene 1 stretch, but better than any other baselayer I've tried. Finally, the 1/2-zip makes it quick to dump heat when you get back to the lift line.
I don't like Powerdry (Cap3/Cap4/R1) as a baselayer. The grid texture feels a bit rough against my skin once there's moisture on it - a bit less comfortable than merino wool. The powerstretch is much more comfortable, even though it dries slower. However, I do use an R1 hoody as a midlayer over the Capilene 1 or Powerstretch 1/2-zip.
I use the 1/2-zip Capilene 2 for high-energy one-day activities in warmer weather - when it'll be my outerlayer - mostly spring/summer touring and summer/fall mountaineering.