1/4 to 1/3 from the front. Middle is great, but I don't mind being off-set a little bit, if I get the immersive viewing angle.
Ive sat way back on a few occasions. While its not ideal, I can enjoy the movie just fine, except in the cases with a very loud projector. Now, that drives me crazy.
I will also go around all of the seating from where the entrance is... to start looking for a seat. Most people start walking up the aisle that is closest to the entrance, and I think better picks can be had by taking the 5 seconds to walk to the other side.