Hands-down one of the most useful pieces of mountain wear is the long-sleeve zip-t. Mountain Hardwear's Advance Lightweight Zip T is state-of-the-art, unbelievably thin and light with excellent durability and serious wicking power, thanks to a channel weave construction.
The Zip T weighs in at 5.8oz, the lightest piece of this kind that we've seen. My first concern was durability, but the Zip T shrugs it all off. If you're truly ham-handed and like to rip your clothing off you might want something thicker, but for normal wear-and-tear and abrasion-resistance the Zip T has no worries.
One nice touch is no pocket; since base layers by definition go on under layers I've never understood why some vendors insist on putting pockets everywhere. The Advance Zip T has a few subtle but truly appreciated touches. A zipper garage at the neck protects your gullet from zipper bites, a longish hem will provide good overlap with your bottoms as well as not leave a seam under your harness and hipbelt, and an anti-microbial Chitosan finish made from crustacean shells keeps the reek at bay.