My suggestions.
AFG Pilatus PC-12
Premeir Socata TB- 10, 20, 21
Geofas Beech Debonair
Anything by Milton Shupe IE: Rockwell Twin Commanders, Howard-500, Beech-18
TB-700, recently added here.
I know you are looking for freeware but for about $10 USD you can download a very nicely done KLN-90 from FS2x.com and some very nicely done freeware Garmin GNS 430/530 and their associated units. They all would add much to a freeware GA plane.
For a blast from the past, take a look at fsdzigns.com. Their connie is nothing short of beautiful.
I don't have a whole lot of payware planes because people keep making great freewares. With some in-expensive enhancements like avionics and gauges from realityxp and sim-flyers any freeware plane can be made as immersive as most paywares. The paywares usuallky have the edge in the gauges and avionics but if you get those things separatly you can use them on freeware plane after plane and not miss a thing.

AMD 2800xp on gigabyte vt600l k7 triton overclocked @ 2.3 ghz, 768 PC 3200, 128 DDR 6600GT AGP, 60 gig,5200 rpm maxtor, 160gig 7200rpm WD, Sony FD Trinitron 19