I think the Diono Radian/Sunshine Kids folding convertible is probably one of the best choices out there. It'll be tough to find information on which is the thinest but given that is one of the few on the market that are designed to fold I'd imagine it to be one of the smallest.
I'm trying to decide between that and the OoBr Clek
http://carseatblog.com/8697/the-newe...ooster-review/ which I think is one of the best looking kids seats out there. My only probably is my kids are too young to use it still be a few pounds.
Diono = compact, foldable, great for planes, ugly
Oobr = best looking, great mechanical design, super safe