SkyC said:
I hadn't thought of the witcher, do you need to have played the first game to get the full experience out of it?
No, not necessarily. It of course helps, but really i'd say before even playing the
first Witcher, go read some of Andrzej Sapkowski's work to familiarise yourself with the world and the lore. The Last Wish is a good book to start with to get a flavour of the world and of Geralt. It might even influence your decisions during moral choice moments. For example, you might think initially support the Scoia'tel, but after reading the books and getting an idea of what they're really like, you might think twice before offering them your aid next time. It will also help you to get an idea of how Witchers tend to favour neutrality above picking sides.
I highly recommend The Elder Scrolls: Oblivion and the Mass Effect games for great role-playing experiences. Oblivion can last up to 100+ hours and its many side-quests are a joy to behold. Do you have a favourite genre? It might help narrow the choice. If you value story above all else and don't mind old games, The Longest Journey is, to me, one of the best PC games of all time and an absolute testament to how character development and interaction can and should be handled in a lavish story.