Just the one? There are quite a few on your polling list I find outstanding...
There was the pleasant surprise of being blown away about how cool (and fun to photograph) Assaj Ventress was...she turned out to be a real dramatic and great looking figure. There was the long wait that was proved well worth it with Leia as Boussh...a figure that just captures the character and really shows Sideshow's strength when it comes to detailing outfits and catching a portrait so perfectly. And recently, there has been the one that has really grown on me with how truly excellent it looks...and that's ANH Han. He's another one I've enjoyed taking a lot of shots of.
Sidious and Palpatine are both amazing renditions of the galaxy's ultimate deceiver featuring two of the most superb sculpts Sideshow has presented.
And one glaring omission from your list as far as I'm concerned is Darth Maul. Still one of my favorites in this collection...great portrait, great outfit and plenty of accessories, including all of those saber variations...really good figure.
But ultimately there can be only one...and up to now that would have to be the ultimate Jedi Master...Ben "Obi-Wan" Kenobi. I still remember the thrill of seeing this figure at ClV and realizing Sideshow was presenting the best possible version of the venerable knight that's this longtime collector had ever seen. He's a real stand-out. My fingers are crossed that Lord Vader is presented with as much quality as his former master has...