If we're talking about official maps, then definitely "Everyone Fights," especially when playing as "Castle." Players with 5 cities in the center of the map are at a disadvantage, and it's not easy to play as any other race on 130% difficulty or higher...
If anyone has completed this map, please share your opinions and strategies, as it's not always possible to win consistently on 130% difficulty.
From the unofficial ones, Gwynn-Ap-Nudd, playing as Necropolis, is challenging for me.
And it's easier to play with HD+... without it, it's much more difficult.