I've got the Sprawl Loosefit rashguard, and it totally rocks, it has so much stitching it will never rip, and has alot of elastic down the bottom so it holds tight and doesnt roll up you when you are on the mats.
I am using a Under Armour HeatGear sleeveless T these days, tight fight and keeps me cool. I have an Atama rashguard that I feel is a little too thick, almost like a wetsuit. I have a Full Contact Fighter one which is also very comfortable and has lasted me several years now.
XFGear. There is nothing on the market that really blends the best of strength, durability, style, customization and comfort. It's one of those products that if you use it once, you will never stop using it. Plus you can design your own stuff! Can't beat it.
I have sprawl loose fit and like it quite a bit. I did not care for under armour as a rash guard. It felt like it was going to rip all the time and once you were sweaty it was damn near impossible to take off.
I have a sprawl loose fit which is great, have not had any problems with it, but i also bought an own brand long sleeve rash guard from the local surf shop for about 1/3 of the price and its going fine, no probs at all
I have the new Nike Pro Vent clothing. Its like their regular dri-fit stuff, but with lil holes in strategic spots, like the armpit areas. Its like Under Armour gear but 10x better. Nike Pro Vent is $$$, thou.