"2112" is a classic, as is "Moving Pictures", as is any of their stuff from the 80s, as is any of their stuff from the 90s... and their stuff from the 70s is great as well!
If you're after heavy stuff, then may I recommend "Counterparts", which was released in 1993. Awesome album, and probably their heaviest in my opinion. "Test For Echo", their last studio album, is also excellent. They're not metal, though, so don't go expecting any. People said they were Led Zeppelin clones after their first album, but after that they were unfortunately labelled as being 'prog', which isn't correct in my opinion. They're a ROCK band, damn it!
Trivia note - after The Beatles, the Rolling Stones and Kiss, Rush have the most Gold albums in the US of A.

And *still* most people haven't heard of em!
Anyway, all of their stuff is cool, but I'm not as fond of the following albums: "Caress of Steel", "Hemispheres", "Signals" and "Presto". Everything else rules, and if you're after an excellent album with excellent lyrics, get "Hold Your Fire".
Oh, one more thing - the singer, Geddy Lee, has a bit of a high voice. Think Mark Slaughter with his boys in a vice and you're halfway there

He's very good, though, and one of the best bass players in the world, if not the best.