Knights of the Nine is not an expansion. So that leaves Tribunal, Bloodmoon and Shivering Isles. Out of those three I picked Bloodmoon because I love Nordic themes and snowy landscapes.
I disliked Tribunal because I dislike "interior-cells-acting-as-exterior-cells." Mournhold felt like a beautiful prison to me. Consequently, I never spent more than fifteen minutes there.
I'm embarrassed to say this but so far none of my characters have set foot in Shivering Isles. In this case it's not because it's a different world space (as a Plane of Oblivion that makes sense) but because, as a roleplayer, I dislike receiving notes out of the blue telling me about something in the game. I want to discover things naturally, as I might in real life. Now as it happens, few of my characters venture down that far, so they do not naturally encounter the portal or have a plausible reason to hear rumors about it. I keep meaning to create a character who has a purpose to go down there but so far I haven't managed it.
So, for me, Bloodmoon wins by default.