Arket T-shirts are getting great reviews. I have their ice crepe T-shirt which is lovely, great quality but very thin.
Uniqlo U crew neck is excellent quality for that price. It is very thick and it keeps the shape very well.
Uniqlo U relaxed fit has more of a raw cotton feel and if you like that it's grand but for me the one I posted link for is much better quality.
Cos t-shirts seem to be good too. It depends what style you like.
Uniqlo sizing is fairly standard stuff. I 'm size 10 in most brands and Medium in Uniqlo for t shirts and leggings. Uniqlo U sizing for other stuff not t-shirts is a bit hit and miss but it tends to be better quality.
In Cos size 10 or 12 is Small. Both Arket and Cos have free returns, Uniqlo delivery/return policy is a bit of a pain. I order when I'm also getting kids jackets (brilliant for the price) or I buy trench coats from them. If you are only buying a t shirt it's not worth it.