Fired all three (M16, AK and FAL) Like the AK for the simple fact that when you pull the trigger you KNOW it's going to fire (if there is a round in the chamber of course). That was key. Found the AK the least accurate of the three. Just couldn't hit stuff. Again, like Der Adler said, it could be the shooter.
Found the M16A2 to be nice. Refined. One thing I liked is the design sends the force directly back. The raised sights on the M16 are there to eliminate the pivot point rifles had right behind the trigger guard, on the M16 the force goes straight back. Sight picture doesn't change in semi-auto. The AK is the best of the old designs and the M16 is the start of a new design. Now see all new types of rifles (from the 70s on) with that same straight throwback, bullpups included.
Found the FN-FAL to be a neat rifle to shoot. Loud, kicks well, accurate. Fun. A lot like the M1. Big round too. Of the three, I found it the easiest shooter, but not neccessarily the best shooter. Again, could be my shooting ability. But it was fun.
Of the three rifles, I definitely think the AK made the most impression on the world. On the flag of Nambia, simple to use, mass produced. You can break the thing down in no time at all. Designed for people who did not really know what they are doing to use. It's also familar to almost everyone in the world. Probably right up there with Coca-Cola or McDonalds.