I have a difficult time fitting tanks, the armholes are always too big, I like the straps to cover my bra, I don't like them too tight or too loose, and I don't like them too long. I've been wearing Soma tanks because they fit perfect but I don't like that they are not cotton.
Yesterday I stumbled on a great fitting, 95% cotton tank at J.Jill. It comes is missy, petite and tall and they even have a 'long version' that gives amble backside coverage. Plus the color range is amazing. I have been looking for the 'right' pink shade to layer under a blouse and found it, on sale. Once I put it through the paces and see how it wears I will go back and get more.
I also picked up a great linen/cotton tee. Perfect fit in the bust/shoulders and not clingy around the mid section and hips. My Mom loved the fit on me so much she tried it and bought two. Bonus, they are also on sale.