Second best is 4 subs, one in each corner. That tends to be easier to do... Of course, I have 2 subs and they are both up-front because I wasn't planning on a 2nd sub when I started my build. I wired the room for it, but based off of looks and layout of seats, etc, and not performance, and ended up with both subs behind my front screen wall.
I still had a nice performance (as in quality of LFE, not quantity since my first sub was plenty powerful without even using 1/4 of the gain) bump acoustically by adding the second sub.
Maybe V5.0 of my theater (it would have to be in a whole different house, and I honestly don't want to think about that now) I will plan for better sub placement.