The Klim suit is probably the best "One Suit" currently available. Either the Adventure or the Badlands. Zipped up with a layer underneath and I'm good into the low 30's. Once it gets over say 95*, nothing is going to be comfortable but the Klim is as good as anythig else.
In fact, I sort of retract what I said earlier. Over 90* or so, I think the Klim may be better than the Motoport mesh, especially if it's a dry heat. The Motoport flows so much air, you can actually dehydrate somewhat quickly. With the Klim jackets, you can adjust them so there's enough airflow to keep your cooling vest working, without sucking all of the moisture out of you. It's sort of like you have your own personal micro-climate.
If you're not moving fast and it's very hot out, NOTHING is going to be comfortable. Nothing.