I think the wild variations I see in GGK fans' ranking of their favorites proves the man's versatility as a writer.
For my money (though I admit to not having read them all yet--and purposely so), Tigana edges Lions for my favorite. Something about it kept tugging on strings I didn't know I had. I wanted to scream the title at several points while reading. Lions tugged too ... but on strings I was already fully aware of.
I didn't dislike Ysabel by any means, but it and Song for Arbonne would rank after Under Heaven on my list.
And even though I probably liked The Fionavar Tapestry the least, I
did still like it. Quite a bit at times.