Here's a few other instances:

• icosahedron.website: Even stronger content policy than mastodon.social. Run by a comrade like mastodon.social.
• anticapitalist.party: Explicitly anticapitalist instance.
• gnusocial.no: Although not a Mastodon server, it is compatible with Mastodon. Home of rms.
• lgbt.io: Mastodon for LGBT with extra moderation and anti-harassment measures.
• mathstodon.xyz: For math nerds, run by AMS (American Mathematical Society). Supports LaTeX.
• soc.ialis.me: For socialists. Mostly marxists. Be aware of tankies.
• octodon.social: Nice domain. Run by a comrade. Not very moderated due to only one staff.

Ultimately, the choice is pretty unimportant. The only differences it makes are:

• your domain.
• the local timeline (if the instance is small, people may communicate within it and have fun there). This is partially the reason why there are so many specialized mastodon server.
• trust (do you trust the owner to protect your privacy etc.?)
• content policy and moderation (all the ones I've listed have good moderation, but you must still be aware that not all servers have that many moderators, so if you pick a low-staff instance, you will have to do some blocking by yourself)
• possibly interface and modding.