Gas piston is the way to go!
I have a Czech (or somewhere in that region) made Daisy that's been a good gun, but it's taken some work to get dialed in (trigger was horrible, spring was LOUD, etc). You can make them mostly silent with a single chamber can, and since it's not a firearm, it's completely legal (so long as it can't be mounted to a real gun). They also don't suffer the barrel length issues of firearms, so you can cut the barrel down as short as 6" and not impact the velocity. Mine sounds like an air brad nailer, and it's just the spring noise.
The high pressure compressed air offerings are interesting, but they cost as much as a nice firearm.