The best Civilization V mods
Gameplay and Total Conversion Mods
G&K Enhanced Mod
Along with the Unofficial Patch (which is required to use it), the Gods & Kings Enhanced Mod touches every mechanic in Civ V. The AI is more rational, barbarians are much tougher, and just about everything has been rebalanced.
One of my favorite, smaller features is its Oregon Trail-like decision prompts, such as "frequent seasonal floods of this river provide unusually fertile topsoil for farming," with options to pay for new irrigation canals, research better ways to protect homes from floods, or let the locals handle it. Really, all you're doing is spending gold to earn a little extra Science, city-state loyalty, or what-have-you, but the prompts add flavor to the story of a city and civilization.
Get it on Steam Workshop—if you don't have Gods & Kings, grab the vanilla version instead. It should be noted that quite a few commenters have reported significant bugs—I haven't been able to replicate them yet, but if you need to troubleshoot, there's a forum for that.
R.E.D. WWII Edition
Unlike the R.E.D. Modpack on the previous page, which just adjusts the looks of units, R.E.D. WWII is a total conversion World War II scenario pack. More than just some maps that look a bit like Europe, it adds features such as capturing and liberating tiles, supply lines, reinforcements, and hotseat play.
This collection on Steam Workshop includes the two mods required, R.E.D. WWII Data Files and R.E.D. WWII Edition, as well as a couple recommended mods, including a version of R.E.D. Xtreme which works with the conversion to scale down the units. Note that most of your other mods will not likely be compatible with R.E.D. WWII.
If ancient history just isn't ancient enough for you, try cultivating a great civilization starting 2000 years prior to the invention of writing. Prehistoric Era kicks things off with cavemen and rock paintings, adding 10 technologies, five buildings, and four new units. City growth and research are a bit more sluggish until you reach the Classical Era, so you're in for quite an epic if you plan to play from the Prehistoric Era all the way to Giant Death Robots.
Civilization Nights deserves recognition just for being as massive as it is—it may be the most comprehensive overhaul of Civ V there is. The total conversion makes significant rules changes (Happiness is most notably revamped) and advertises over 50 new buildings and wonders with custom artwork, dozens of new units with their own models, new policies, a restructured tech tree, and as many tweaks as there are stars in the night sky. Well, not that many, but a lot—the details can be found in its Civilization Fanatics thread.
Unfortunately, Nights hasn't been updated since October of last year, but if you can get past any bugs or unfinished bits you may encounter, it's a unique way to experience Civ V.
A Mod of Ice and Fire
If George R.R. Martin killed your favorite character in A Song of Ice and Fire or HBO's Game of Thrones series—and, spoiler, he probably does—you can design your own hexagonal fate for the fantasy series' warring families. A Mod of Ice and Fire is the best GoT mod I know of—all 10 new leaders have unique traits and special units and buildings, the religions, Great People, and spies have been renamed, and the technology tree appropriately ends in the Renaissance Era. It also comes with a Westeros and Essos map, a just-plain Westeros map, and a map of The Known World.
Grab A Mod of Ice and Fire on Steam Workshop (requires Gods & Kings).
Yet (not) Another Earth Maps Pack
You've got to have a good earth! There are tons to choose from, but I like Yet (not) Another Earth Maps Pack for the variety it offers in one package. There are several maps here—including Europe and Mediterranean maps as well as the whole of earth—and some added setup features.
With the mod loaded, select "custom game" to get to its custom start screen, where you can select a version of earth, and adjust city-state placement and resource distribution.
Strange and Historical Religions
The names and icons of religions are just for flavor, but its still a treat to found the Brotherhood of Nod, Church of Star Trek, Pastafarianism, or any of the other fictional and real religions and philosophies included in the Strange Religions pack. Objectivism vs. Scientology, fight! Actually, please don't.
If you prefer more historically significant faiths, the Historical Religions mod will give you fun, hard to spell religions such as Tlateomatiliztli, Pesedjet, and Forn Siðr, complete with custom icons and their own Civlopedia entries. If you need even more ideological fidelity, grab the Historical Religions Schisms Addon too and divide Christianity into Roman Catholicism, Eastern Orthodoxy, Oriental Orthodoxy, and Protestantism.