Lots of 1/2 inch hardware cloth! I plan to enclose most of mine in hardware cloth, as well as putting goat fencing along the bottom. I think that will be 100% predator proof.
For something that is just predator resistant, I think garden fencing at least 4 ft high with netting over the top should do.
No gates/door in the fencing (The coop sits in the middle of a 25' x 50' yard, which is attached to a building that has a door we can enter the chicken area from)
There are 2 electric wires that run across the top (was having an issue that my dad insisted was a raccoon attack so he added a second, stronger electric wire - I think one meant for a cow).
We stretched netting across the top (something very similar to what is on a volleyball net) - the problem wasn't a raccoon but an owl. We had tried stringing fishing line over the top but that didn't work.
We haven't had a problem since all of these were installed.
Our second coop, is our chicken retirement coop, so not as worried about security, but we haven't lost a single hen since we installed the electric wire across the top of the fence. Our fence is only 4 feet high on this one, the electric isn't too strong, a few of us have been zapped when we forget. We also installed a automatic door since there is not netting over the fenced in coop - but it hasn't worked in a year and still haven't lost anything.
We have a flock of Guinea fowl that free range, they can fly out of the second coops fenced in area and roam the yard. They slowly get picked off by a hawk, fox, etc during the day. Perhaps that is what keeps our chickens in that coop safe.
Thanks so much for the advice! The reason i was asking was because i lost a duck today to a predator in a pen i thought was predator proof because i have never lost any ducks in it.
Thanks again!