Rush-Hemispheres! Without a doubt.
I bought it on a whim soon after it came out based on a review in one of those record club booklets they used to have in the 70's (remember those? Get 6 albums for the price of 1 when you join the Columbia record club). It described the "six part title track", and that sounded so cool to me, that I had to try it. I was listening to Aerosmith, Led Zeppelin, Peter Frampton at the time and I was ripe for discovering something new. I put it on my turntable and listened to the 3 minute long intro and was blown away by the music. I had never heard such tight, intricate, complex music in my life. Then Geddy started singing and I thought that was the most bizzare voice I had ever heard, yet I liked his voice.
I was immediately hooked on Rush, and remain a huge fan to this day. They really opened my eyes to the world of progressive rock which led to obsessions with Yes, Genesis, Jethro Tull to a lesser extent, and finally to DT.
To this day, Rush and DT are my two favorite groups.