I have to consider myself an expert in this field,I live in Miami and between April and October Cool Max,Dri-Fit,etc.. shirts is pretty much all I can wear.Now pretty much all these shirts are polyester but there are differences,I have about 7 pairs of Nike Fit dry t-shirts and there absolutely can't be anything better to wear in hot weather than these because even though it's polyester they are thin so you automatically just feel comfortable in them and when you sweat they don't collect the sweat like a cotton t-shirt does.Adidas ones are pretty good but I prefer the Nike ones.Again it's all about how thick they are as I have a Quick silver t-shirt also made of polyester however it's thick and just has a different feel to it like you'll be hot in it,I haven't worn it since February and don't expect to wear it again until November or December.Old Navy also has some pretty inexpensive ones called Rec Tech or something like that that even though are not as nice as the Nike ones are pretty good as well,they had them on sale one day for like $3-$4 and I think I bought like 6 of them they only had like 3 colors otherwise I would have bought more.I've never bought any Under Armour ones but I've noticed many of them are not as thin as the Nike ones.Even if I go shopping for Polos they have to be of some sort of cool max material.