I voted for CTTE. A no-brainer. Because it is the album on which YES found their sound. Because the title track was a progressive step foward in pop music. Because AYAI still moves me. Because the weakest track (Siberian Khatru) is still better than most of the stuff recorded by most of the prog rockers. And because the album features the best line-up of YES writers.
But having said that, the recent tour has changed my perception of Relayer. Where once it would have been my 3rd or 4th favorite album, it is now a close second. Not just because GoD has truly come alive, after all this time, on the current tour, but also because this experience took me back to the album, and Sound Chaser -- perhaps the most radical and progressive track YES ever recorded. And then To Be Over, which so many people here seem to love, and which, as the years pass, sounds better and better. Like SC, it has a radical structure, even more so than the material on CTTE.
I would pick Fragile, The Yes Album and GFT1 next in a Top 5, but I wouldn't know how to order them. It depends on the day of the week. But these 5 albums are, in my humble view, clearly the cream of the crop.