TL:DR – The best Mastodon server to use is one that has a focus on your main interests and where there are lots of others users, also one that is well established and has good policies, a strong admin team, and is financially sound.
If you’re new to Mastodon, then have a quick read of my Mastodon FAQ to get an idea of what it’s all about. Once you’ve done that or if you’re ready to dive in, here’s a list of the instances/servers that are most commonly being used by the people I followed on twitter who added a Mastodon ID or link to their bio and I could find via Debirdify.
Because they’re all federated you can see what’s happening on other instances, but you might want to opt for an instance in your area to begin with, or perhaps one where you know contacts and colleagues are already active. It is relatively straightforward to migrate from one server to another and take your followers with you, but it’s probably best to choose a well-established server that’s been around for a while.
I first tried Mastodon in August 2016, only a few months after the first system launched, but I didn’t sign up properly until November 2019 and I opted for the most obvious instance at the time, the general Mastodon.Social, so you can find me there @[email protected]
I should point out that there are currently more than 12500 Mastodon instances with almost 6 million users (just over 2 million active within the last month). You can search for those other Mastodon instances here.