Arc'teryx Delta is where it's at when it comes to insulating snuggly fleece. The Delta comes in three different weights: the Delta LT is a snug lightweight baselayer, the Delta AR is a midweight high-loft layer, and the Delta SV is a high-loft midlayer jacket.
The Arc'teryx Delta LT Zip Pullover makes a sweet baselayer under a softshell jacket or you can pull it over your tee on a cooler fall day. The Delta LT is made with Polartec Classic 100 Micro Velour, and features a small grid pattern to trap heat when worn under a jacket. This moisture wicking, breathable fleece top has a killer warmth-to-weight ratio, and keeps you comfortable during aerobic activity. Also check out the Delta LT Jacket, which is like the pullover, but with a full zip for super efficient venting.
Next in line is the Arc'teryx Delta AR Zip. The AR part stands for All Round, like all year round, which is when you can use this jacket. This mid-weight baselayer features a 1/2 zip for venting, a trim fit, and a longer cut, all of which make it totally rad for layering. The Delta AR is made from Polartec Thermal Pro with a high-loft and a grid pattern to help regulate your temperature and keep an excellent warmth-to-weight ratio.
Finally, the Arc'teryx Delta SV Jacket is the most hard-core of the Delta club. It's probably club president, or at least treasurer. This plush fleece is their warmest and most protective fleece jacket. The Delta SV features Polartec Thermal Pro construction with a high loft that traps heat, wicks moisture, and is breathable. Wear the Arc'teryx Delta SV as a stand-alone chilly weather jacket, or as a killer baselayer in mountain climates.