RTR 6 by itself is really old, PE (platinum edition) is what followed, then there are 2 more "major" mods of it that are ExRM (extended realism mod) and Imperator Metro & Naval. There is also a "mini" mod from AqD. Both ExRM add AqD's mods are still being worked on but not by the RTR team.

The RTR team released a mod based on the invasion of Iberia by Hamilcar Barca (RTR VII: The Iberian Conflict) which is a lot of fun, though, i never got to finish it before my video card passed away

They are also working on RTR VII: Fate of Empires of which you can see the previews (and all the mods said) in http://www.twcenter.net/forums/forumdisplay.php?f=26

out of experience, i've only been able to try PE and ExRM, and i enjoyed them both, but the first one had some issues that were mostly fixed with ExRM.

I doubt any other mod has as much depth as EB, so it will be easy to be disappointed with other mods, but they are no less because of that.