Patagonia Capilene 2 Zip-Neck - Long-Sleeve
Lightweight, quick drying, great for aerobic activities when the mercury drops. I have had mine for five years and when they do wear out, I can recycle them through Patagonia. Can't beat a good performing baselayer that also has a conscious for the environment.
Zipper helps regulate body temperature. Only wish they would make short sleeve versions with zippers as well!