I love Target's Performance sheets a lot. They have a double elastic on every corner so they never pop off and there's a little tag on each side that says "top/bottom" or "side" which is surprisingly helpful when making the bed!
We've purchased from the Tahari brand a few times and have been satisfied. Also, I recommend giving Jersey Sheets a chance (Target has some inexpensive ones that are amazing). They are almost as soft as Flannel, but seem to be more breathable.
I love my bamboo sheets from Purple. They are so soft and durable. They are also super breathable so it it's cold, they'll keep you warm and if it's warm, they'll keep you cool. Hypoallergenic too which is a plus!