I say that my first anime was Zatch Bell, and after seeing it several times before, I say it does hold up. While it certainly didn't have best animation, and the voice cast tended to be either obnoxiously high-pitched or inappropriately uber-deep(I honestly prefer the English dub's voices), it did make up for it in other departments. It has a diverse cast of characters from across different parts of the world, and it does have a very interesting justification for the Mon concept as well as the competition-type of story. It also plays with common elements within shounen series, like how while the protagonists do get stronger and gain more abilities, so do their enemies and manages to keep the battles going in a case of strategies, tactics, and cleverness instead of only just brute force. And it is really funny and there is some good legitimate drama to be found in it.