Well, the train is the most obvious answer, because it takes you from point to point probably the quickest. There's a Eurostar train if you want maximum comfort and ease, although I personally had no problem with the IC/ICPlus trains and even the regional trains. Eurostar trains will have more facilities for luggage right at the doors so you don't have to lug it all the way into your compartment; some cars in the IC trains may have room for luggage at the end of the cars as well.

Here's the Trenitalia website in English to book trains:

http://www.trenitalia.com/en/index.html