Best base layer I've worn was the circa 2012 Pearl Izumi Transfer and Transfer Lite with Minerale fabric. No longer made but some new/old stock remains -- unfortunately Nashbar has only women's sizes available but at only $10 these are incredible bargains and would make great gifts for friends and loved ones. The remaining unsold stock still usually sells for close to full retail, around $50 each. I'd still consider paying that much, the fabric is that good.
The current PI base layer is not as effective or comfortable. I can't recommend it at the full $50 pop. Even my $10 Champion Vapor tees are better.
The older PI fabric was much more effective at wicking moisture and resisting odor, due to the anti-microbial properties. It could be worn more than once without washing, great for campers, hikers, touring cyclists, anyone who doesn't have access to laundry facilities every day (but can be hand washed and dried easily).
The fabric was stretchy and fit snugly yet was easy to slip on and off. Back in May after I was hit by a car and my shoulder was dislocated and broken, this was the only pullover shirt I could wear for weeks. Anything else had to be button or full zip. But the PI fabric stretches generously yet regains its shape.
It was temperature neutral -- it felt warm to the touch on cold days, and cool on warm days. When soaked with sweat on cold days it didn't feel clammy and chilly. I even wore it under good shirts on hot, humid summer days and it prevented sweat printing through the outer shirt. It didn't feel muggy and uncomfortable.
Best undershirt fabric I've ever worn. I've heard similar comments from local friends who also have PI baselayers from the 2012 or so batch, including the kind with wind-resistant fabric on the front.
The current PI baselayer is almost skin tight and doesn't stretch. While technically my size (I'm 5'11", 160 lbs), the medium was actually painful to pull on and remove -- mostly due to a shoulder injury that has taken months to recover. After one wearing I gave the shirt to a friend who's a little smaller, between a small and medium, so it should fit him better.
And the new PI baselayer fabric doesn't wick as well, despite being thinner with mesh in the underarms and back. It was designed to wear in summer under a jersey, but felt clammy. It lacks the odor resistant Minerale fabric and needs to be washed after every wearing. It's not suitable for camping, hiking, bike touring, etc.
I got it for only $13 in exchange for a review, so I wasn't too disappointed. I've I'd paid the full $50 retail I'd have returned it for a refund.
Pearl Izumi confirmed to me by email the older Minerale fabric is no longer available. Some high end camping/hiking outfitters still carry baselayers with similar anti-microbial properties via yarn impregnated with silver, if I'm recalling correctly.
Meanwhile I'm sticking with Champion Vapor tees. Not as snug fitting, but almost as good for only $10-$15 depending on short or long sleeve versions. Note that the Champion wicking tees that are not specifically labeled "Vapor" (a reinforced hem around the back on the neck) are not quite as good and are more prone to staining. The Vapor tees also launder better, including minor oil/grease stains from chains, etc.