Sicily. Apart from having some of the best starting infantry in the game, they also had the Muslim Archers and Norman Knights. Under my control not even the Mongol Hordes stood a chance against me. Funnily enough, though, France are the only faction I've reached the Americas with. Out of all the games in the series I've spent the most time playing Medieval 2. It might be my favourite game of all time.
Sicily are great fun to play with. I remember having to make a lot of early moves in order to get the economy going. I would usually get a foot holding in Tunisia and then focus on moving east through Byzantine territory. The Northern Italian states would act as a buffer during the early stages. I thought the American portion of the game was disappointing. I would have preferred more map out east to incorporate the Persians.
Rome 2 looks fantastic so far, but I don't think my computer will be able to run it when it comes out. It can just barely run Shogun 2. Speaking of, I finally sorted the drivers for my video card out during the week so I've been playing Empire: Total War a lot. Completely different game and I'm still getting used to it, but I'm really enjoying it.
My laptop can barely run Medieval. Did you ever play any of the mods? I can never get them to work and rarely feel like spending the time to try and figure out the mechanics in them.