Congrats! You've definitely seen many changes in this business over 20 years. I've been around for just over 20 as well.
We are married to cPanel/WHM. Customers find it easy to work with, so we stick with it. Do you work/offer all the panels all at the same time? Is it something your customers specifically ask for as part of custom solutions, or you just enjoy playing with different software? Coming from your experience, how do you find these panels compare? If you had to list your top 3, how would it look?
We have been working with cPanel, but as India is price sensitive market we shifted to DirectAdmin and now most of our shared servers are on DirectAdmin however we still have cPanel servers for the Fans.
Since we're more in the web design world than the hosting world, we've found most of our clients are using hosting companies with their own control panels. We still work with a number of cPanel installations, but these days it seems to be less and less.