Pretty simple really. What draws you to your favorite character? What about them speaks to you in a way the others don't? Can you relate to them, or are you inspired by their unlikeness to you? Do you long for their special abilities? Does a specific life circumstance, weakness, or theme resonate with you in particular?
I'll start with what is probably a lot of people's favorite character: Wally West. As simple as it sounds, I find Wally very relatable. He's serious without being dreary and depressing. He's sarcastic, and has a sense of humor that isn't overbearing. He's a symbol of hope, but is much more personable and down to Earth, both as the Flash and as a civilian (the latter having a blue collar flavor). His loneliness and sensitivity as a child is something I think is very easy to tap into from a nostalgic angle, as is his reverence for Barry and feelings of inadequacy as he grows. His sense of justice is also more natural, like Barry's; not driven by tragedy. His ability to think and move at super speed are very appealing, especially in that bright red costume. There's so much freedom and control embodied in that blur. While part of me would love to have that ability, another part of me feels like my mind is already over-thinking at a rapid pace constantly. For example, I can relate to when he expresses frustration at shying away from debates because he's covered the angles in his mind a thousand times. It's a kind of arrogant thing to say, but I think plenty of people feel that way about certain topics. Sometimes the mind can work on its own so much, the rest of the world can feel redundant.