I don't know about perfect in the sense of having nothing you or I would dislike, but I find that the best mods for these games take a long time to perfect/release. An example for RTW would be Europa Barbarorum, which meets my criteria for a perfect RTW mod. EB2 isn't out, but looks like it will be even better. Broken Crescent for MTW2 looks great. I'm not sure if it's out yet, I stopped following it when I found out the first release wouldn't work for 1.3 or Kingdoms.

What are you looking for in a mod?