While I generally (if secretly) agree with Zouave, I have to disagree about the best mod. Mind you, I've only played 2 so far - Plutarck's LWC and Dreifels' "final solution to corruption-by-distance" so I can only comment on these two.
Firstly, both mods improve the game-play over the plain vanilla Civ3 (not hard to do given how half-baked it is).
LWC does bring in lots of new units and buildings. Many people may like this added realism. I find it distracting and over-complicated, my main criticism. The modified tech tree looks more like a spider's web than a tree. Many of the buildings don't have icons on the menu and they appear at the bottom of the menu instead of amongst the other buildings (easy to overlook). Also there are sometimes odd crashes in later stages of the game.
Now that I've played Dreifels' mod, I much prefer it - simple and effective. Does a brilliant job of correcting corruption-by-distance (even on multiple continents) and the AI's tendency to swarm the map with those annoying size-1 towns on every available tile. These were my 2 main gripes with the official versions of Civ3. Dreifel has done some good things with the Wonders pop-up screen and a few other niceties with the tile graphics too.
For those odd people who actually like corruption, they can still play Dreifels' mod and just choose not to build any "state palaces" (effectively just extra Forbidden Palaces). Corruption will be as bad as you like it.
And finally, the mod did not cause any civ3.exe crashes, at least in my experience. I was playing under Patch 1.17f by the way.
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