Initial impressions:
octodon.social: one of the largest instances. Lots of French and Japanese, seems kind of a neutral meeting ground between other instances. Local timeline has lots of personal, nerdish things, not much politics or news.
oulipo.social: mostly quirky, with interludes of serious posting within the constraint of not using "e"
mastodon.xyz: very large instance. Local timeline is very twitterlike: short toots, not many rants. Lots of French, lots of linux.
witches.town: lots of French, gender quirks, playful. Few men. It's like the good parts of queer twitter, but with added witchiness.
a.weirder.earth: Small. Full-on geek-outs, lengthy toots on philosophy and languages, weirdos weirding at each other.
mastodon.social: huge, no real sense of community, way too busy to use the local timeline to identify the zeitgeist. Basically twitter, but with a code of conduct.
wandering.shop: small, friendly, excellent code of conduct, not many writers or fans yet but definitely a great place for SFF.
Of them, mastodon.social is my least favorite. It's very busy, it doesn't have the cohesive zeitgeist that other instances have. As a discovery tool to find other instances, though, probably great. Think Grand Central Station.
octodon.social for me hits the sweet spot of a community where the zeitgeist is visible, bonds with tons of other communities, etc. It's more like being at a party than being in an airport.
I'd advise:
Find one general instance: large, lots of subcommunities, a bit too chaotic to really be able to primarily use the local timeline. For me, that's octodon.social
Then, look at who ends up in your personal timeline. Lots of people from fibble.wobble? Go check it out. It's likely to be smaller, have a more identifiable zeitgeist, a cooler local timeline.
Next least favorite: mastodon.xyz, for the same reasons. As a clearinghouse between instances, probably better due to its smaller size.