Your basic, useful, long-sleeved crewneck T.
Midweight interlock. It's thicker, soft, luxurious, and warmer. You won't wear it above about 70-degrees, but it's the best choice when it's cooler and you don't necessarily want to layer over it.
FIT: On the snug side. They stretch a lot, but if you wear a large T-shirt or dress shirt and you want one of these to cut you a little slack, buy up a size, or even two. Machine washable and dryable, but we always suggest air-drying.
I/Grant wear a large T-shirt and go for XL or even 2X in these. I like loose clothes.