What is the easiest to load shotgun?
I would like clarification on the question. I know it seems simple enough, but do you mean load, reload, or cycle?
Simply loading, I say a break barrel shot gun. You can have loose shells in your hand one second and 3 seconds later, (if the gun is unloaded) you already have a solid weld to the cheek and you are a VERY dangerous fellow....
Reloading - if the break barrel has an ejector option, then its pretty fast still but if not, pulling the shells out really slows you down. (I hate that about my Double Defense). With my Browning Superposed, ejecting the shells over your shoulder when reloading is almost as fun as the shooting. That said, I would likely give the edge to the standard pump style guns.
As for cycling, my Dad could cycle his Model 12 faster and more accurately than any semi that I've ever seen, so I guess that comes down to personal feel and practice.
I have to say though... I'm curious... WHY do you ask? Shotgun shells are fairly large and easy to handle. It's not like trying to put loose 22LR into a tube or anything. They are all quite easy to load in my opinion.