Truth be told it's tough to pick a favorite, I like several items that have been released with many of them from 30th Anniversary line!
I'll give an honorable mention to both the Imperial Sentinel and Darktrooper from POTF era because the designs are cool, they're just too limited in POA for my taste.
In my top five has to be K'kruhk, T'ra Saa, Shaak Ti(TFU), Revan, and pre-Cyborg Grievous. Simply because we would never have gotten those types of candidate prior to 2000's or even now. This list is just a very small sample size though and if I listed everything I like it would fill a page.