Given my usage, wants and needs there are two answers IMO...
For purely the most fun at a range for the least bucks the Norc M4gery wins hands down. Uses AR 10rd mags (obviously), drinks Magpul kool-aid, can be upgraded easily with triggers, safeties, rails, uppers etc. and you can be shooting with it, a couple of mags and 500 rounds of ammo for under a grand brand new.
Away from a range and in a gopher patch, I'd have to say the HK SL8. The most accurate out of the box non-res black rifle I've owned/fired by far (haven't tried a Swiss, but they're in another price zone anyways), can take AR 10rd mags with a magwell swap, is ambi out of the box, has G36 upgrade options (if that's your thing) and you can be zinging varmints with it for just over two grand.
Rifles I've owned to base this on: Norc M4 (2), Stag lefty AR, Tavor (2), non-res FS2000, HK SL8, Mini-14 (3), JR Carbine, non-res CX4. I've also tried an XCR and an ACR (briefly..very nice rifle, but pricey).