I am planning a trip to Europe. I will have one month. I would love suggestions for an itinerary. We arrive in Rome and fly out of Madrid. What are the "must sees" and how much time would you suggest in each place? What is the best transportation from place to place?
You'll need at least 10 days for Italy, 14 would be better. Then add 5 days for Paris, 4 days for Barcelona and spread the remaining days between France, Spain and Madrid. Must sees in Italy are Rome (3-5 days), Florence (3-5 days), Venice (1-3 days).
Transportation within Italy - by train, from Italy to Paris - fly, from Paris to Barcelona take the night train or fly, from Barcelona to Madrid take the night train.
From Barcelona to Madrid you can now take the fast-train as well, it can bring you to Madrid within 3 hours. Flights are also still an option, and probably cheaper, although with check-in and luggage handling, and the travel to and from the airports, it will take longer than the fast trains.
For Madrid I would suggest 4 days, 2 for the city, and the other two to make daytrips to Toledo and Segovia.
For the rest I can agree with t_maia, but for me Pompeii, would be on the must see list for Italy (a daytrip from Rome is possible, but long), and if you would like something else than the cities, I would suggest Cinque Terre.
If you have some time to travel around in Spain, I think that seeing the Alhambra in Granada is a must-see, just like the Cathedral and Alcazar in Sevilla.
For the most ideal would be to have two weeks in Italy, and two weeks in Spain. Direct cheap flights are not the difficult the get by, so connection between the countries are good. The rest of the countries will still be there on a next visit. So don't rush things too much on this trip.
What about Marroco. If you have a few days left I would definately take the ferry from Algeciras. Its quite an experience at itself to go from Europe to Africa. You could visit the Maroccan city of Tanger and even move a bit south towards Chefchouen (a beautiful Marrocan village with a pleasant old city and good and extremely cheap facilities).