I agree that C2C has quite too much going on. Needs some clean up to filter out irrelevant buildings and units. Just way to cluttered. It does have some interesting concepts and runs a bit better than it did in the past.
Been a long time since I tried out RI. I played the old version of it - forget the name. However, it just runs too slow for me even on the best computers. Otherwise, I'd consider it the best of the comprehensive modpacks.
@Thoapzi:
I encourage you to take Lemon's advice on BUG/BULL and BAT. These are related mods that provide unaltered gameplay improvements to the normal game, if you wish to play the normal game - which I highly recommend for now. As for the graphics in BAT, you may not think you want it, but you may be pleased once you have it. Blue Marble can be added as well to BUG/BULL install for improved terrains and such.
(Note: BAT is stand-alone version of BUG/BULL+Graphics. It has everything you need for unaltered gameplay. BUG/BULL can be installed stand-alone, but I recommend installing in Custom Assets w/Blue marble. That way the mod stuff is "just there" when loading BTS normally. Saves generated can be run by other players with no problem. Great option for forum gameplay)
As for other mod's, probably rehash here:
RFC (and modmods) - Nice alternate way to play the game. UHV(unique historical victories) provide a nice and different alternative to standard Civ victories. Some are fun and quick, while others make for a longer game.
Fall From Heaven (FFH) - Probably the most popular mod outside RFC. Fun fantasy game that you can spend 100s of hours on. Mod Developer is a lead designer over at Stardock now working on their own 4X fantasy game.
Pie's Ancient Europe (PAE) - Very nice era focused modpack centered on the ancient world. Lot's of new era specific units, buildings and objectives. Also, includes some unique scenarios. A very well developed mod that is still being worked on today.
Legends of Revolution and Rise of Mankind (plus modmod) - These two modpacks I haven't tried in sometime. I find them similar in some ways, but also in the vein of C2C and RI, i.e., basically large comprehensive modpacks that serve as expansions to the game. Both mods have been quite popular in the past.