I like 2112 (the song) more than Cygnus X-1 or Hemispheres (the song). So it boils down to whether the 2-3 other good songs from 2112 overpower the 2-3 good songs from Hemishpheres and Farewell to Kings. It's a close call, especially if Xanadu isn't your favorite Rush song.
By my count I get 21 minutes of awesome on 2112 (Overture/Temples/Oracle/Finale) is 10 minutes, Bangkok, Twilight and Something 4 Nothing add another 11 minutes. A Farewell to Kings gets 19 minutes of awesome (title track, Xanadu, Closer to the Heart), while Hemispheres gives you 18 minutes of awesome (with Trees / La Villa and perhaps Circumstances). Further demerits from A Farewell to Kings for Madrigal and Cinderella Man which I find worse than Lessons/Tears.
But then again, I like Caress of Steel, especially The Necromancer, so what do I know?