Dead Rising 3

To get an S rank on a district, you need to earn a certain amount of points before the end of all 5 rounds. If you're using some of the later character costumes, it won't be hard at all to rack up huge points and get S ranks easily. I found the best characters for S Ranks were BB Hood, Sigma and Regina.

In fact, I got all of my S ranks by playing solo on my first try (except the first district). Here are some general tips to rack up huge points, as well as approximately how many you'll need for the S Rank on each level.

I found that on rounds where you needed to destroy certain objects (Big Bombs, Generators, etc.), BB Hood was the best character to use. His Special will wipe out literally any object in the way and crank out like 50-60k points per object. On levels where there were bosses, BB Hoods Y button attack is also extremely effective, as it not only stuns the boss, but also deals out very good damage, even without the POW upgrade.

So, in terms of how many points you need, I found this for the levels (again, these are just approximations, so it could be more or less).

Ingleton ~741k
Central City ~1.09 million
Sunset Hills ~1.01 million
South Almuda ~ 2.09 million

Again, these numbers aren't set in stone, that's just around what I got for points when I S ranked those levels.

User Deranged Asylum was kind enough to provide videos of himself completing these districts with S Ranks so everyone can have a kind of idea of what's needed to get S Ranks. Much thanks!

Just a side note to the great solution above: You don't actually have to clear the district to get the S rank. I ran out of time on Sunset Hills fighting the Shadow Boss, but I still had enough points to get credit for the S rank.