I wonder what Total War expansion people liked the best? Here I'm talking about the expansion campaign, not any patches or additions to the vanilla game that came with the expansion.

I always thought BI was clearly better but was called on this on the M2TW forum. Here's the - opinionated - case I made for it:

Quote Originally Posted by Simon Appleton
Well, your mileage may differ. But the scope of BI was far greater than either MI or VI. In MI, you had only two factions and a very simple (not to say hypothetical) strategic situation. In VI, you were stuck on a damp insignificant island with some squabbling primitive factions that lacked even decent cavalry. By contrast, with BI you got a full RTW sized map and a great choice of varied factions covering an earth shattering period of history. I personally think BI was almost a RTW2 rather than an expansion. Rome's fall has always captured my imagination more than the its republican period.