R.E.D. Modpack
One of the weirdest parts of Civ has always been unit scaling. It’s very hard to see the realistic difference between an infantry soldier and a tank. Thanks to R.E.D., though, you can now see the difference in size a bit more clearly. Most of the R.E.D. mods are actually quite nice, but this one is perhaps the most necessary for the average player. Being able to eyeball a unit and understand its strength relative to others just by size will help you to make split-second decisions with a bit more data.
Smart AI
While Civilization might have the appearances of having a smart artificial intelligence behind the scenes, the truth is that the game is actually kind of dumb. Instead of using intelligent moves, the computer just receives a number of bonuses. Smart AI seeks to do away with that by boosting the game’s AI to allow it to make smarter choices and more realistic moves during gameplay. If you are tired of dominating the computer opponents, this might be the best mod for you. It’s still in rough shape, but it can change the way the game is played.
Prehistoric Era
This conversion mod greatly extends the length of a Civ game by putting players back in the Prehistoric era. Think of it as prequel to the main game, forcing you to take your civilization through early discoveries like fire and the wheel. It’s actually a great replacement for the early stages of the regular game, giving you more to do while you develop new technologies. While perhaps not as polished as some would like, it’s a good way to lengthen your games while making the earlier years seem much more relevant than in the traditional flow of the game.