Close to the Edge is good? I only have Fragile and The Yes Album from their back catalogue. The former's pretty awesome, the latter is really mundane.
I'm trying to collect as much progressive-rock as I can at the moment. I get the idea that it would be my favourite genre of music if I were to become more well-versed in it (I like King Crimson, Pink Floyd, Jethro Tull, Yes etc. Emerson, Lake and Palmer are a horrific testament to the potential of overly-pretentious music to utterly fail when it is coupled with bog-standard songwriting).