Just yesterday I replied to a post where a similar question was asked, so I hope it's okay if I just copy-paste my answer:

The best Mastodon instance is the one that aligns with your interests and values the most.

• Are you interested in tech? There's a couple of tech-focused instances.
• Are you some flavor of LGBTQ+? Some instances do a better job at keeping out bad actors, and you can be around like-minded people.
• Are you interested in gaming? Movies? Art? Writing? Game development? Home improvement? Gardening? Activism? Memes? News?
• Maybe you're interested in stuff happening in your country or local area?

Why? When you're looking for new content, and new people to follow, the local and federated timelines of your instance are a good way to do so. Your home timeline includes all the people and hashtags you followed yourself, and their boosts. The local timeline includes all the posts and boosts of everyone on your instance. The federated timeline has all the content that everyone on your instance is following. (Of course you can always follow anyone you like, but I'm making a point about ease of discovering content relevant to you here.)

For this reason, just joining a big, general-purpose is less useful, since you're just going to get a hodge-podge of random things in these timelines. Perhaps you don't mind, but I feel like it's good to point out this feature of the fediverse, as some people might not know, or realize this is a thing.

How? Okay, of course this is silly to recommend without giving you some way to look for these instances. There's a couple of directories that allow you to search for them. Looking for some briefly came up with https://instances.social/, https://mastodon.help/instances and https://mastodonservers.net/. Also note you can migrate your account from one instance to another, taking your followed content and even followers with you.