T-shirt and boxers. I tried sleeping in the buff for a while, and while I enjoyed it, I discovered that I have skin that is more oily than the average person (especially on my back) and I didn't like having to change/wash my sheets extremely often.
Though this is only tangentially related, I will say that if I'm in my sleepwear (T-shirt + boxers), but I'm not in bed, I will almost always put on pajama pants. I absolutely love the feel of comfortable pajama pants and I don't like the way the chair/couch feels on the back on my mostly bare legs.
Also, if it's late in the evening or it's a weekend/holiday, I will shamelessly go out in public in T-shirt + pajama pants to make quick runs to the store or go grab some food.