I have tried Ubuntu, Fedora, Puppy.
Ubuntu is the most friendly to Windows users trying Linux out for the first time.
Fedora is a bit more technical, not so user-friendly to first time Linux users but a really good OS as well.
Puppy is one I like because it runs on relatively low system requirements, so for older machines it does very well where other distros wouldn't.
Other very popular ones are Knoppix and Mint. But my suggestion would be Ubuntu until you get more familiar with Linux, then you can branch out to some other distros (distributions) that may or may not have already been mentioned. Good luck !
