It's a bit hard to choose just one... But I really like Flite Streaks... Something about that mean tail just grabs my imagination. My preference is for the original TF kit version. The ARF flies fine but the fuselage is a glass jaw, it just breaks too easily. It's easy enough to build from plans. It really responds to a few modern improvements. A slightly longer nose and a stretch in the back makes it easier to fit a 4oz tank, and improves the flying characteristics. Slower controls make the Streak easier to fly.
The Sig Skyray 35 would probably be my next recommendation. The kit falls together very quickly. The plywood wing ribs tend to be heavy and introduce various undesirable side effects, but even built with all the kit parts, to the plans results in a really decent profile sport plane with enough capability to do OK in stunt. The design responds really well to having the plywood ribs replaced with balsa, and substitute full ribs for the half ribs. I also add fixed flaps to the trailing edge of the wing. Plans show fairly fast controls, moving the pushrod to the inside hole in the bellcrank makes the plane easier to fly.
If you want to step up to a flapped plane, Sig's Twister and Banshee are good planes too. The Banshee looks a little funny, but just changing the tail and shortening the nose can give it a completely different look. The Twister is more popular, and easy modifications can easily be found on the forums which make it a really competitive plane. The pro's and con's of both planes when compared to each other works out to almost a wash. The Twister comes with light ply fuselage doubles that should really be replaced with 5-ply birch, and the flat wingtips are a bit unfinished. The Banshee comes with real ply doublers, and wingtips, but can look funny and doesn't have quite the fan base. Truth told the Fancher modifications can be applied to both planes with similar results. Both are easy to build and fly well.
Regardless what you choose to build, you can't really go wrong.