For me it's definitely the original, but since that one doesn't count, I have a few others who do rank quite close to each other as some of my other favourites.
The first one is the Sinnoh League Victors theme song. At the time, I was pretty annoyed when I found out that there was yet another season for the fourth generation, because I really just wanted to move on to Unova, since I had yet to experience anything with that region yet, but I was met with this super catchy theme that I eventually started getting in my head a lot, and it kinda helped a bit. Turned out that it was a great season anyways, so that's why it remains in the top for me.
Close to that would be "I wanna be a hero" and "Unbeatable", since both of are honestly really good song, and those were 2 of the seasons that I actually watched through. I heard the first one for the first time when it aired here in Denmark, although, it was in Danish after all. I wouldn't really say the quality was reduced though, and I fell in love with it. The first time I heard "Unbeatable" though, was when I watched the Lucario movie. They were far behind here in Denmark, and when I discovered that I could watch the episodes online, it wasn't long before I was right on track, and I started it all off with the Lucario movie. I loved the song, and loved the season it was the theme song for, so not much else to say really.