Every morning, I roll out of bed, toss on some clothes, leash up the dog and go for a 30 - 60 minute walk around my neighborhood. As you can imagine, I'm not really strutting around in designer fashion. I carry a roll of poop bags and a baggie of liver treats. It's not very glam! And I definitely don't want to wear anything that I'll cry over if it gets dirty or sweaty. So I own a lot of t-shirts.
My favorite tees right now are the Old Navy Women's Perfect V-Neck Tees from the misses line. Old Navy's crazy vanity sizing makes it possible for me to wear the XXL with room to spare. These are not high quality, wear like iron tees. I like them because they're light weight and airy. At under $10 each, I just bought a bunch in different colors and barely made a dent in my credit card. They're not perfect but they get the job done.
If you're looking for something a little nicer, I like the entire Supima cotton line from Lane Bryant, especially the cotton shirred sleeve top. The cotton really is super soft and the colors are pretty. If you like your tees to have a slightly longer sleeve, you'll want to check this one out.