Sugar/sweets. I crave it and stopping cold turkey makes me want it more. I've been this way since I was little and even though as I get older, I don't want the very sweetest of things, I still want something sweet.
I think about this question frequently. If I had to pick one of my biggest weaknesses I would choose public speaking. I do not do well when it comes to speaking in front of a bunch of people. I don't really care for the all eyes on me scenario. I am working to get better at it, but it is something I have struggled with for a long time.
My biggest weakness would probably be being too afraid of failure, especially in the social department. I feel that if I open up to people about my life/secrets I'll be ridiculed or harrassed. I've been a worst-case scenario thinker throughout my life, so I often think about what could happen that is bad and not necessarily good.