You guys put me to shame... My favorite is a 4" x 2" butt plug. It's my favorite because I've learned to maneuver it (hands-free) to hit my prostate perfectly. Without a doubt, my best solo sessions ever were from this tiny little beauty.
My biggest is 6" x 5" and before my current BF, it rarely got used - maybe once or twice a year. Now I use it to get prepared for him, which is still not that often.