This depends first and foremost on where you want to go.
If you're planning to visit a single country, you can choose a One Country Pass for that country. If you want to visit multiple countries, the Global Pass (valid in 33 European countries) is your best bet. Do note that you cannot purchase a One Country Pass for your own country of residence, and that with a Global Pass it is only possible to travel in your own country of residence twice (to leave and return at the start and end of your trip). See this page for more information about this.
Secondly, you should choose a Pass that matches the number of days you plan to use the European train network for. The travel days are offered in a wide range, starting at just 4 days and up to a maximum of 3 months.
Thirdly, if you're looking for a little more comfort and luxury, a 1st class Pass is the way to go. If you're watching your budget or don't care much for luxury, 2nd class is the more affordable option to see all of the places on your bucket list.
Finally, you can choose to travel with a Mobile Pass or a Paper Pass. A Mobile Pass offers you paper-free travel and you don't have to wait for delivery, we'll send you everything by e-mail. This makes it a great option if you want to start travelling right away. If you want to travel with a Paper Pass, just select 'Paper Pass' in the checkout. Delivery will take approximately 10 days, so make sure you order your Pass on time.