24mm has been my most-used lens for the last 18 months or thereabouts, I've always been a wide-angle kinda guy. So I'm tempted to say 35mm, but I have almost no experience with this focal length, though I have a few of them.
My least-used lens, historically, is the sad and lonely 50. Recently somebody convinced me I ought to shake up my photography by shooting something different, so I've pulled out my 50s and am going to shoot with them exclusively for the next year. I was quite disconcerted by the tight framing I got when I first tried to shoot with one a few weeks ago, but by the end of the day I was enjoying myself, forcing myself to see differently. I may even go so far as to acquire a 50mm Macro.
Next year, 135!