I'd say try Denmark, I just tried them they are fun.

The beginning can be easy mostly do to rebels surrounding you. Your first war will probably be with HRE so you have a nice large power to challenge you. They have some of best early infantry in game and even in mid game can go toe to toe with most as the AI rarely upgraded beyond armored sergants.

Your only decent calvary other then stardard knights will be whatever chapter house you get, Templar, Hosptalliers, etc.

As technology advances Denmark starts to fall behind so more challenges as time progresses.

Once gunpowder hits then things get interesting.