If you have a newer disc (2008 or later) or a new download patched to 1.4b, the game is fine, in that it is playable. If you are only to have one mod, any of the supermods (GWX, LSH, NYGM, WAC) will do. I also recommend SH3 Commander, which is not a mod but an external tool which will add as much to your campaign as one of the others does to your game.
Other than that, everything is icing on the cake. But what icing it is. I am currently running more than 60 mods.
