Choosing 1 is difficult, so I'll go with my standard top 10 favorites...
10. Death Star Droid Freeze Frame
9. Luke or Han in Stormtrooper disguise
8. Ishi Tibb
7. Biggs Darklighter Freeze Frame
6. Luke Bespin w/Removable Hand
5. Darth Vader w/Removable Helmet
4. Death Star Trooper
3. Wuher COMMTech Chip
2. Sandtrooper
1. Chewbacca as Boushh's Bounty Freeze Frame