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.