Wanderlust World Wanderlust World Icebreaker is the silky-textured Cadillac of the Merino wool world. Icebreaker pieces are fashioned of pure, New Zealand-sourced Merino wool and are luxuriously soft, gentle on the skin, odor-mitigating and breathable. Icebreaker has excellent heat-dispersal qualities (the same attributes allow moisture to dissipate quickly from the skin), yet still stands up to some serious backpacking punishment. You can wear it for days (weeks, if necessary) without needing to wash it.
Of course, it's also distinctly spendy---though most folks who've worn it in extreme conditions think it's well worth the extra cost.
If you're looking for high thermal performance at a more moderate cost than Icebreaker, Montbell's natural/synthetic blend, Super Merino Wool, might fit the bill. It's water repellent (helping the body stay dry in wet conditions) while remaining mostly smooth and itch-free. Super Merino Wool is antibacterial, which makes it resistant to most bacteria-caused odor, and is generally reviewed to be softer than SmartWool.
Unlike most pure-wool pieces, Montbell's anti-shrinkage technology helps the pieces stay true to size through scores of washing machine cycles.
Capilene is Patagonia's fully synthetic base-layer fabric. Designed for sweaty activities and soaking-wet conditions, this polyester fabric comes in a range of four machine-washable fabric formulations (each geared to a specific type of condition and/or activity). Gladiodor manages odors in the fabric.
Best of all, it's green---Capilene is made of at least 50 percent recycled content (and is itself recyclable when returned to Patagonia).
If you're interested in options aside from wool, synthetics or blends, Terramar offers a 100 percent silk baselayer option. Silk is a natural insulator, and Terramar weaves it into a thermoregulating, shape-retaining fabric that has a UPF rating of 15+. Wearers' main complaint is that the delicate fabric snags easily, so it's not suited for the rigors of some backcountry adventures.
If you can find it (the Swedish brand can be difficult to find in the U.S.), Woolpower makes a top-notch base layer that's 100 percent Merino wool. The company has been developing the fabric technology for half a century (testing their work in Sweden's brutal winters) and has finessed the designs to the point that they're almost seam-free.