There had to be a Trek character in here, and as much as I prefer the original Shatner series to the newer stuff, I think that the original show is much more of an ensemble of characters. Kirk is great, but he needs Spock to be his super logical counterpoint, and Spock needs Bones to be constantly pissed at him. I think Data stands up as a strong character completely on his own. As this is a Star Trek character we're talking about, I'm sure there are many readers scoffing and shaking their heads. Well, this is a list of MY favourite characters, based on the criteria that I like them or enjoy them, and that is all, so get over it.
Data is an android who desperately wants to be human. He studied at the Starfleet Academy, just like his human colleagues, and graduated to become an officer. He is always inquiring about what it means to be human; trying to understand jokes, tears, love.
He does have his low points (the all too frequent Sherlock Holmes episodes, which I kind of love as well as hate) but for the most part Data is a really strong character who develops throughout the series and into the film series, always learning more about what it means to be human and becoming more human himself.
Brent Spiner is pretty amazing. Imagine getting your skin painted like that on a VERY regular basis for a VERY long time, while also sticking to that android performance. Sure, Arnold Schwarzenegger is a convincing robot because he's huge, but I think Spiner is the king of making me believe that he is not human, and he does it all without just looking stupid. Data is one of the strong points of Next Generation and one of my favourite characters of all time.