I rather enjoy fire arrow. It seems to be right between the mods that are overpowered and the ones that are way underpowered. It has a lot of features I enjoy, including new armors that actually go well on female characters, cattle pastures and wheat fields that can be seen on the world map, and optimized menus. The best thing about it, though, is the balance between ease and difficulty; it's challenging but not impossible, beatable but not stupid.
I guess my bias is a little unfair, though, because the only mods I've played on this list are Fire Arrow, Native Expansion (III), and Ranger's Apprentice. Ranger's apprentice was downright annoying with the super speed lords and way overpowered units. Also, the game play is way too fast for my liking. I got to level 30 in one night and had a force of 400.
Speaking of Rus XIII, though, I can't wait till it's finished. Even the beta was probably one of the best mods I've played, while I have to admit I have no idea how to pronounce any (I mean ANY) of the city, troupe, or character names.