I'm going with BI.

I think some of the campaign map features were better (less sophisticated, but better game play) in MTW, but RTW wins on the balance, and BI is an improvement over vanilla.

The dream for me is RTW's slickness with an AI that make it necessary to scramble for every possible strategic advantage (night battles, ambushes, forts, choosing terrain, weather, etc. all these potentially cool features play little role in my games)

When it comes down to it, 2/3 of my battles are against rebels and most of the rest are sieges.

I remember feeling great trepidation when launching an attack in MTW. The enemy was guaranteed the defensive advantage, usually had decent stacks if not multiple stacks in its border cities, and a defeat might leave a gaping hole in your defense.

In RTW, I honestly never fear sending a stack into enemy territory. They're unlikely to meet an AI stack of the same quality, and if they do, they will be on defense.