There are definitely some good mods listed here. These are mine that I use pretty much every game:
-Ethnic units (Definitely a must have for me, its simple but gives an added "spice" for each civ. I prefer it over R.E.D. because my laptop is low end and I have to play in windowed mod and R.E.D. are too small)
-Historic Religions (Gives a lot more options for religions than the base 10, more variety IMO. Also tweaked for each civ to have a preferred religion)
-Infoaddict
-Some of WHowards mods, such as Notification options and trade opportunities
-I use the Unofficial Patch only when using the GEM mod. Using GEM makes it almost a different game at times (in a good way)
-Cultural Diffusion (like in CIV, each tile has its own % of culture and they can flip. Also, options can be tuned to allow military units capture tiles, etc
-Cultural Capitols (there are cultural "elections" that happen every set number of turns and if your civ wins, a city at random is chosen and you can receive either a permanent or temporary bonus, such as increased production or workers, just for a couple examples
-The Visigoths (fun new civ to play as)
-More City States (just adds more CS to the game but from what ive read, it can cause conflicts at times since some of the CS added are also part of some added civs, Tyre comes to mind)