What's the difference between favourite and best in the case of a matter like this one? Well, for me the first YES music I properly heard was YESSONGS at the tender age of 11. What an impact at that age! And, the album sleeve artwork really connected to the music with a massive impression at that age. So for me that is the best, or favourite. I suppose favourite, since it does seem that TALES has better artwork, in terms of the detail and appropriateness of the art themes to the music as noted above by others. So there is the difference between best and favourite I suppose.
But I'll always prefer the images to YESSONGS. I've always thought since those young years that Anderson's lyric in ROUNDABOUT, '...mountains come out of the sky and they stand there.' is reflected in Roger Dean's image in the YESSONGS artwork of those cone-shaped mountains coming down from space onto earth. ROUNDABOUT is of course contained in YESSONGS. It's just a pity that the effort that went into the YESSONGS art couldn't have been as detailed and as well done as the artwork for OCEANS. Well, these are my views on the subject!