Native maps are the best balance wise. Every class is useful on any native map. Some are of course better on certain ones.
Ruined Fort - Cav can be used even with the big hill, just dont fight on a slope, Archers are fine of course, Infantry is the most important part of a team here, they become important in the end
Field on the River - not much to say to, very balanced
Village - Quite balanced, depending on the teams, if they camp or not.
Nord Town - balanced for any team, camping possible but can easily prevented by a glitch
Port Assault - very balanced, attackers just have to be quick in the beginning
Ruins - balanced, good for cav and archers and in the end it is more about infantry fights. also about teams and tactics
However, seeing what I wrote to the maps above can also be applied to many custom maps. I do not know any name of the custom maps but I love some of them. Just an open field is of course not balanced, cav has quite a high advantage there. I guess the server just needs a nice map rotation and it should be fine.