I've heard others say Relayer is their best.
Some of this stuff isn't so easy to find, esp. something like Tales, a double sided CD taking up shelf space, where they might if they're lucky sell one unit a month!
I'm having the same problem trying to find Tubular Bells.
Relayer is good,but I don't think it really exemplifies the classic "Yes" sound.
Jon Anderson of Yes made an album with Mike Oldfield.