Ted King

 "It is said that Ted King's real departure was due to a heated argument between him and Shannen Doherty.

Note, this is just speculation that was leaked years ago, but none of them have addressed the issue or clarified anything.
There are also rumors that Ted asked for more involvement and when he couldn't come to an agreement with the producers, he decided to leave right at the end of the first season.
It's hard to believe that the character of Andy has become one of the most beloved characters over the years and he could easily be in the top 10.
And if Andy had lasted another two or three seasons, his character would have developed more and perhaps his ending could have been much stronger, more impactful not only for the Halliwell family but for the entire fanbase."