Freewill! If I had to pick one song to show what Rush is all about it would be this one. All the boys going on all cylinders, just amazing. Not to mention the great lyrics by pratt.
2112 of course, but whats a more interesting question is whats the SECOND favorite. There Im going to say Lessons. Love Alex's acoustic guitar playing, and Geddy's singing is fantastic. Great uptempo forgotten tune.