the best is definitely a difficult term...
I've played a lot of the different mods now.

XGM is professional, slick, fairly similar to Vanilla in a lot of ways, and dead easy to install. But it adds a lot to the gameplay and has a lot of classy touchs. It's also very stable. I've never had a CTD with it, and that's saying something.

EB is hands down the most detailed and complex of the mods... it really is quite different on almost every level. It's definitely for playing the long game, for those who want detail and reality.

And at the other end of the brilliance spectrum is Blue Lotus. This is my favourite for the atmosphere... it's a fantasy mod set in the far east. Blue Lotus is a bugger to get going and install, and has about 3 little mod patches and so on, so it's not easy, but it is worth it.

Other ones to look at may be Zhaguo, The Fourth Age and Rome Total Realism are all good, solid mods.