Thanks for all the input. It's nice to have so many good choices.
Right now it's a horse race between Italy and Portugal, with Portugal starting to pull away. As much as I love Italy, I have seen plenty of it from Lake Como in the north, small cities and towns in the middle, and Sicily in the south. I have seen less of Portugal, mainly Lisbon and Porto.
Portugal won.
If you really want to take the train, I would say Switzerland. France and Italy have far too many strikes during tourist season. Plus a Swiss Travel pass covers trains, buses, boats, and some funicular trains and cable cars, and entrance fees to over 500 museums and castles.
A rental car is much more convenient however.