Choosing a Mastodon server is ultimately a matter of trust in the operator.
The server should be in line with the purpose of your activity and preferably in the country or region where you live.
How to choose a Mastodon server
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Mastodon is designed to be a decentralized social network, and it's strengths come when it's decentralized.
I'm sure you've always thought that choosing the most well-known, popular, and popular service would be a good choice.
With Mastodon this is not necessarily the case. Rather, there are many cases where it is better to avoid such servers and choose a medium-sized server, or a smaller server, or your own server.
@noellabo@fedibird.com This applies to ALL fediverse platforms, so in terms of SNS don't limit your choice to Mastodon, there are also Misskey 
, Pleroma 
, Friendica 
, GoToSocial 
and more, they can all talk to each other with different favour of UI and UX, just look around.
Fediverse is designed to be decentralised, self-hosting if you can, put your data in your hands. Don't know how to do it? Check out https://fediverse.express , it is a good means for the beginners.
@noellabo how to choose a mastodon server:
don’t, use pleroma instead 
@sophon I am often told by supporters of Pleroma and Misskey that I am writing this for Mastodon.
I use Pleroma... I'm using Akkoma right now, and I'm also running two public instances of Misskey while supporting the developers of Misskey and spreading the word about PeerTube and Pixelfed.
It's good to endorse products you support, but doing so out of context can hurt your image.
@noellabo ik I was semi-joking
@sophon Yeah, I thought so! But there really are a lot of them...
@noellabo@fedibird.com @sophon@plma.plus.st I personally benefitted from your article for Misskey, I am grateful for that, so I am fully aware about your support to other projects. Fedibird is truly a splendid fork of Mastodon👍, therefore I understand you are in favor of Mastodon. My only hope is that people can use fediverse rather than Mastodon network, as such new comers would know it is not just a boxed "Twitter alternative" but a broader world with greater possibilities to be explored.
The more users Mastodon's server has, the more effort and money it takes to maintain it, but it's not designed to make money, so the balance is getting worse.
Mastodon's answer is to divide it into smaller parts and manage smaller parts in order to solve the problem of large-scale services like Twitter, which are becoming too big and causing various harmful effects.
Guys, don't get discouraged if mastodon.social is slow. The solution is to move to a medium sized server that works better!