Vallejo Matt acrylic coat for airbrush is, in my opinion, perfect - for airbrush, that is. If you prefer to use brush, Vallejo have a line of coats for it, too. Or the line of acrylic coats from the can, if you prefer it this way... Alltogether, Vallejo are the best acrylics I've used. :)
Pollyscale is pretty good, but it goes bad relatively fast, with time. At least flat coat can go bad and leave white spots on your paint. Online shops can jolly good sell a 5 years old paint to you, if they need to get rid of old stock - that's what happened to me, with the following catastrophy. :) Good thing it was just a small boat I've tried the coat on.
Haven't seen any bad vallejo paint as of yet, though, so I would advise it.