Linux is free because the man who started it all is not a man who lived only to make money and more money. He's a man who believed that the best things in life can be free. This philosophy is what has kept Linux free and good all these years. There is no best flavor. It's more a question of what you want to do with Linux. If you just want something to surf the net, then I suggest using Puppy Linux. The distros you mentioned are variations of the same thing with different desktop environments. Some desktop environments have more features and, consequently, are also heavier, thus slower. Some desktop environments have just the barebone features, making them lighter and faster.