I will also vouch for the Pico chair I own. It is a good compromise between small folding size and comfort. Note that it is definitely a compromise however.
Another chair that I am surprised not to see mentioned yet is the
Travel Chair Easy Rider. It is slightly wider and more ergonomic than a typical Walmart folding chair (WFC), built of much more durable material, but actually slightly shorter when folded than a WFC. It is the only one I know of that hits this sweet spot. Most of the more premium folding chairs are dramatically heavier and bigger than a WFC and not well suited to van travel.
I have never been able to understand the use of chairs like the Kilos. They require constant abdominal muscle engagement to avoid tipping over backwards. If I wanted a workout I would just stand. I sit because I want to relax!