Yep, Country Gentlemen were great and to me their best album was the self-titled one which had Traveling Kind, House of the Rising Sun, Casey's Last Ride on it as well as a bunch of other great bluegrass and folk songs. That line up included Doyle Lawson, Bill Emerson, Ricky Scaggs and Mike Auldrige along with Waller. My favorite CG lp, although an earlier incarnation with Eddie Adcock and John Duffy put out some great stuff.
I agree a Bill Monroe anthology would be a great starter.He's still awesome to listen to.
Also Tony Rice did a great one with a bunch of younger and older players called Tony Rice Plays and Sings Bluegrass. I can't recommend that enough.