ONE’s custom ATO suit is the best I’ve used for hot temperatures. It’s lightweight and breathable, and features zippered vents under the arms and in the crotch which are a huge relief on warm days. And to top it off, you can get it all custom fitted and design which also helps control temperature and overall comfort when your suit fits correctly.
I’ve tried various Sparco, Oakley, and K1 suits (I live in Texas) and the K1 suits I had were the lightest weight and most breathable; however, in terms of durability the Sparco and Oakley win hands down.
I normally run an Oakley suit but when the temps get high (or I feel I need to shave every ounce of weight) I break out the K1 (custom ordered).
I really like the OMP KS-4 suit. Its affordable and lightweight (Level 1).
But recently I thought I’d give an OMP Summer-K suit a try. Its not CIK rated, so you can’t race in it. But it’s essentially a single-layer cotton overall, styled like OMP’s kart suits. I use it for practice days (non-official) when its just me at the track. It fits over your street clothes and is very comfortable on hot days. Best thing is I take it home and throw it in the washer and dryer with the rest of my clothes and its good to go.
That said, its safety value is about the same as jeans and a denim long-sleeve shirt. So keep that in mind.