What do you mean by t-shirts, exactly? The big shapeless ones, or just plain knit tops? I got this one last year from Boden and I love it: http://www.bodenusa.com/col.as.....38;col=COR
The fabric is dense and has maintained its shape well. I like to wear it in the summer with a patterned skirt or layered over another shirt in the spring.
I admit I enjoy looking at the tshirts on threadless.com, and I'm actually wearing one now...but the catch is that I'm wearing it as lounge gear. It does nothing for me on top, just makes me look even more rectangular. I gave up on tshirts mid-college. I just found other things that looked better, and I realized that wearing silly pictures on my body was probably contributing to people thinking I was still in high school.
When I do yoga, I prefer something sleeveless as I tend to get quite hot. Other than that, I can't find any reason to wear them.