There have been many different Pokemon songs used for the theme. The United States alone has at least five different songs. These included the first theme, Pokemon World, Pokemon Johto, born to be a winner and I Believe. The first theme song is one of the most memorable ones.
Pocket Monsters also had at least 5 opening music that it used. Of course this was only in Japan and it was not released at all in the United States. There was also the Pocket Monster Advanced Generation which had at least 5 of its own opening songs. No matter what song it was, all of them shared a very distinctive sound that related to the audience. They were very catchy and were remembered.