I really love the Alternate Ranks system in Dark Heresy, with a special place in my heart for Rank 0 alternatives. What are all of your favorites?
The Cyber-Mastiff Handler (Tech-Priest, Book of Judgment) is my favorite, though the Verispex Adept is also pretty amazing. Something that completely changes your style of play and adds a fun level of character complexity. The Maletek Stalker is also super cool; nothing quite like a heretical tech-assassin powered by a Warp reactor.