Which Europe Rail pass to choose?

There are a lot of places that sell the passes, but it's all the same basic pass (actually there are two passes - Eurail for non European citizens/residents and Interrail for European citizens/residents, but you only qualify for one of the two). This is like the situation with airline reservations - you can make the booking at many, many sites (e.g., Expedia, Kayak, ...). The e-ticket you end up with is the same for a particular seat on a particular flight.

You should read the information on this site regarding rail passes. This has much more information about whether a pass is better than individual tickets, selecting the right pass, using the pass, and things to avoid than there is room for in an answer here: http://www.seat61.com/

Conley has some great suggestions.

I might also add the suggestion of visiting www.raileurope.com. Their site will clearly explain all the different railpasses available. I generally travel with the Eurail Selectpass, which allows a certain number of days of travel within a larger period (usually two months), and limits travel to several contiguous European countries. It is better than the unlimited-use Eurailpass, which can tempt you to try to go everywhere and see everything, which you can't do. By limiting your travels to a few countries and spending more time in each, you will have a much more meaningful and pleasurable experience. I've met young backpackers who rattled off a huge list of countries and cities they saw in just a month or two. "What did you see?" I asked them. "Train stations, mostly," they replied.

Almost every source will charge you an overnight shipping fee for your railpass, whether you request the service or not. One that doesn't I found, is AAA Travel, if you don't mind waiting a couple days until the pass is shipped to your local office from AAA's supplier. (All railpasses sold in the U.S. seem to come from French Rail's New York office.)

One thing to remember: Have your pass validated at the station before you climb aboard your first train. If you don't, there's a big surcharge to pay if the ticket-taker has to validate it on board the train.

i'd say Italy. there is Rome, Naples, Florence, Venice (easily worth a bypass to), the section around Lake Como is constructive, and that i've got heard that Tuscany is amazingly eye-catching.