My vote goes to Rush. Although they were not part of the original prog scene, and some may argue that their later work is not fully progressive, all their albums are laced with clear progressive tenancies, and they are without a doubt the most prolific progressive rock band, consistently putting out strong albums every few years since 1974. Plus, they arguably bigger now than they have ever been. Looking forward to their new release in 2012!
My top 3 would be:
- Rush
- Yes
- Jethro Tull (although Ian Anderson never intended JT to be a progressive rock band-- their string of 70's albums are certainly progressive and almost flawless)