Surprised no on has mentioned the Viper M1 yet. Uses the slightly primitive F1 design, but man did they crank the build quality up to 11 on those things. Probably more work went into the valve retention screw on those than an entire BE Avenger.
The ICD blowbacks are of course another favorite of mine, so different than anything else out there. I wish ICD had used that valve and bolt layout in their electros.
And the Golden Eagle was on of my Unicorns. Took forever to catch that elusive beast. In the great paintball dictionary if you look up "overbuilt" you'll find a picture of a GE.