I wonder how many people in here really mean these!
My honest ones, are
Best: RNC / Painful Bodylock
Worst: Armbar from mount. Ugh..... I'll never know what my problem is. One time I stepped on a guys nose, and ever since then I've been afraid to do it properly in friendly sparring. Armbar from Guard on the other hand, is my easiest submisson of all besides the obvious guillotine.