I think this is a brilliant album (I think side one is among the best opening sides on any rock album) and there are several very strong contenders here, but I ultimately went for Tom Sawyer. Probably the obvious choice, but I consider it a sonic masterpiece - it seems astonishing that just three musicians created such a layered sound. The musicianship is also top notch, with the middle section providing a perfect illustration of Rush's abilities as a power trio, as the guitar solo is great, but so is the bass work underneath and the drumming that follows is the icing on the cake. Limelight would be a close second for me.