Tempted to say Riker because he saved the universe in Best of Both Worlds. Jellico can suck on that! But I choose Worf because he's so obsessed with his duty. Just because he gets beat up every episode doesn't mean he's not taking care of tons of security threats offscreen.
Worf. Well, he should have been, if Picard had stopped telling him no. If Picard/Riker had taken Worf's advice every time, at least 80 episodes would have been resolved before any s&%t hit the fan. Samaritan Snare, for example. Data is #2. Everyone else had bouts of incompetence.
I chose Data What would be an interesting poll though would be who was the best Bridge Crewmember who sat in the Helms seat when Wesley left. How many Helmsmen/Helmswoman did they have. It seemed like after the rotating Chief Engineers, they had rotating Helms-People.
I went with Worf- he was serious about his duty and even when off shift he constantly sought ways to improve himself (the Holodeck Calisthenics programs for example). Of all the others, IMO only Deanna Troi was a waste of space. Her role sounded good on paper when they created the show, but they could not really do much with her character. Either she was too powerful and would give away the alien subterfuge unless something negated her, or she just helped disturbed crew members privately. Since everyone in Roddenberry's new universe was near perfect, she just did not have much to do. What she did do was good, but not worthy of a bridge position...
I voted for Troi, the show rarely used her right but she has one of the most unique and useful abilities. In Nemesis she even became a living cloaked ship detector.