Cos everyday t shirt (the ones at £19.99)
Arket heavyweight
Uniqlo as above, but- be warned the quality and weight of the fabric seems to vary wildly. I bought one in store, loved it, ordered the same one online- unrecognisable in terms of thickness and quality, despite being the same product.
I also agree with the poster about the Uniqlo v neck ones- I ordered a couple of those as well, and use them for cleaning up, they look like men's vest material.
M&S Autograph - last year they had some lovely colours as well as black and white. Really good quality.