It was supposed to be one reservoir, yes. Thats what I get for writing forum posts at 2 am...
It's been a while, but I believe Massilia was the hardest 1 reservoir game due to the high prosperity and flat coast (thus fewer palaces to build palaces), or Londinium due to the high prosperity and high population. I think Massilia was harder. The shacks are a major drag on the prosperity rating.
No military, Sarmizegetusa was the hardest because the rocky terrain, badly placed farmland, and attacks from all sides, it makes it difficult to build a stable city and fight off the enemy. If you want to, the challenge can be done without walls. I recall in one of the maps, 1 soldier bugged out and sat in a corner until my (relatively early) city fell apart due to instability and lack of immigration. If this happens, you may build a fort to get rid of it and keep it, but never use it unless the enemy bugs out. Also, I did it without building any towers.
In the 1 granary challenge, I also found Tarsus difficult because the only food source is fish and fish loses a lot of its efficiency if the cart pushers have to travel. That may also have been because I was not good at the challenge yet. Massilia was harder because it is extremely difficult keeping 2 or 3 types of food in one granary without having the main food drown out all the other foods while still providing enough food for your entire city.
EDIT: On the No Starting Funds challenge (Same as the No Handouts Challenge), the hardest maps are the ones with lousy exports and early military requirements. Carthago is actually fairly simple due to wine exports with natives (as long as you can fight off the Carthaginian armies with a fort or two of Javalineers. Destroying the city isn't that expensive. Caesaria can be a bit tricky because there isn't very much to export and some military is required fairly early. The military maps are the more difficult maps because you have to build a stronger military early in the game. It has been a while, so I don't remember which maps are the hardest on this challenge, but I would assume the later military missions are the hardest, especially those which require you to import iron or weapons for legionaries. I believe Sarmizegetusa was difficult due to the large early invasion, but they also have natives, so you can quickly get out of debt. Basically, if you get out of debt and you can win the map, you've beaten the challenge. Also, any map that has required early imports (like iron or weapons), lousy exports, and early strong military encounters.
On the No Exports and No Starting Funds, I feel that using any of the taxation tricks pretty much defeats the purpose of the challenge, so they should not be used.
On the One Granary Challenge, though I remember Massilia as being the hardest, many of the large cities had their own challenges with food supply. I forgot to mention it earlier but Damascus (I think. Desert and Egyptians.) was also quite difficult, as the farmland is separated and fairly thin, leading to an effect somewhat like fishing. It contends for the hardest map, though in different ways from Massilia.
[This message has been edited by Zarquan (edited 07-13-2017 @ 07:20 PM).]