I think it depends a lot on the opponent style and size. Also if you are on top or bottom.

Basically all techniques are good if applied in the correct moment. Who agrees?  

Efficiency is an interesting question. Efficient in what exactly? Energy conservation, submission or control? I'm specifically not a sport student, so my goal in Gracie self defense Combatives is to conserve energy, gain control and then submit. The Gracies have a great saying, "Not all moves work on someone, but at least one move works on everyone."  :D

Favorite...totally different question for me :)

My favorite move personally is a double leg take down. It's not a ground technique, but it's my favorite way to get people to the ground. It allows me to get into a side mount which has a lot of control and gives me a lot of options.
Favorite Submission: Kimura, cause it's fun. You give a nice back adjustment to your partner for free.   :D
Favorite Mount: Twisting Arm Control. Cause it lets me control someone much bigger than me and I can go right into taking their back and/or arm lock.   Favorite Guard: Elevator Sweep. Leverage leverage leverage! Favorite Side mount: Elbow Escape. Awesome way to create space and get the bad guy into your guard. I love it all though.