I love Ralph Lauren's simple v-neck cotton tee's. And they are super cheap!
I also like the Mossimo shirts at Target. They're long, which I like, and cheap, which is good. I think white t-shirts need to be replaced frequently, since they dull in the wash and lose shape quickly. I'm so hesitant to spend big bucks on a t-shirt because mine get too short and too wide/loose after several washes.