This may seem a bit corny, but I most enjoy listening to homemade CD of church music sung by our church choir. It just makes me feel good to hear our church's choirs perform. Perhaps this is an offshoot from the fact that my dad is a retired minister. I just love to sing along. I can't seem to get that music out of my head.
In college, I used to listen to "Moving Waves" by Focus while studying for exams.
When I used to have my own office at work, I often played a bunch of different R.E.M. CD's while cranking out software. It's just not the same when you wear headphones and can't sing along.
I guess that's why I enjoy my multi-hour drive to/from WDW. I can crank the tunes on the car stereo and sing at the top of my lungs.