I've taken all the responses (so far) and according to this unscientific survey (but just as reliable as Arbitron ratings), here are the top XM channels:
1. 60's on 6 (13 mentions)
2. 70's on 7 (12 mentions)
3. 80's on 8 and The Virus (8 mentions each)
4. XMX and MLB (6 mentions each)
5. 50's on 5, 90's on 9, The Blend, Big Tracks and Watercolors (5 mentions each)
6. Flight 26, Top Tracks, The Loft, Lucy, Soul Street, Escape, BBC and Radio Classics (4 mentions each)
7. America, 20 on 20, Deep Tracks, Fred, Squizz, BPM, XM Pops and Air America (3 mentions each)
Keep the responses coming, and I'll keep the tally going.