The Nike ACG Dri-FIT Mid-Layer 1/4-Zip is easily my new go-to for outdoor training. I picked this up specifically for trail running, and it handles the demands of the trail perfectly.
The fit is spot on—it is completely true to size and moves with you naturally without feeling restrictive. It is the absolute perfect weight for cool morning runs; it provides just enough warmth to take the edge off the morning chill, but the fabric is incredibly breathable so you don't overheat once your pace picks up.
I especially love the addition of the thumbholes, which keep the sleeves locked in place and add that extra bit of coverage for your hands before you warm up. If you need a high-performance, durable mid-layer for the trails, this is worth every penny.