IMHO, the best choice is Federal Premium #1 buckshot 2 3/4 inch tactical loads. Forget 3 inch shells. Shot recovery is quicker after the smaller load and you get one extra shot. #1 buckshot is the optimum balance between pellet size and number.
As far as bet gun, HK Bennelli M-1 is the only autoloader to trust with your life. Remington 870 rules as a pump.
WW1999
As the owner of a Benelli Super 90 Defender, you may want to keep in mind that it's recoil operated, which means that the felt recoil is actually a bit greater than that of a closed breach (i.e. pump or single/double barrel), and it's very sensitive to light or even field loads.
Mark
I diasagree that it is the only one. The old Browning Auto five is also reliable as is the Remington 11-87. I personally would choose a Remington 11-87 if it were set up like a Benelli.
The 870 is the best ever, but the Mossberg and Winchester 1300 are perfectly usable.
I live in a rural area and keep a pump loaded with 2 3/4 inch short magnum BB loads. I would probably only use it outside so overpenetration is not a problem. I like the BB load as it is heavy enough to do damage yet has enough pellets to insure a hit. Number 4 buck would be about as good. 00 Buckshot is an awsome load but only 9 pellets leaves possible holes in the pattern.