I liked the Bespin Luke(Though they should have labeled him Luke Dagobah fatigues) mainly because he can hold a gun. That was one of my frustrations even at 5 years old that the original Luke could not hold a blaster. Anywho, I will go with the before mentioned Bespin Luke, Yoda and the AT-AT driver as my favorites. There are more but those are my top 3. Honorable mentions go to Bespin gown Leia, Snowtrooper, and the Bespin Guard.Luke Bespin, AT-AT Driver and either Snowtrooper or Yoda.
I considered the Luke Bespin to be a huge upgrade over the original (Farmboy) Luke, but thought Han Bespin was much weaker as a figure than the original was...go figure.