What I would grow if I could only have 20:
Austin roses, Kordes roses that Zuzu mentioned, and Belinda's Dream, Pink Diamond, drift roses, Easy Does It, Rainbow Sorbet, Distant Drums, Earthsong, Quietness, McCartney Rose, Seminole Wind, Julia Child, Autumn Sunset, Memorial Day, Sunsprite, Jeanne La Joie.
That doesn't leave many Austin or Kordes roses, if you can only have 20. There are 20 Austins and 20 Kordes that I would want to keep. Actually I can't even fathom stopping at 50. There's just so many good ones out there!