Mastodon is the best social media alternative right now. It's a mature and relatively stable software base, that is open source. It allows for editing posts, dumping your data, moving to another server if your sucks but still being on Mastodon.
Post News would be my second choice right now. They've added editing of posts. I like the long form option that's always available. It's run by someone who (at first glance) seems rational, although Andreesen is also an investor and he's gone a little off the rails lately.

Blue Sky is theoretically interesting, because of it's theoretically federated approach which will theoretically be open source.

I think the restricted invite approach has made it seem trendy.

But it's run by someone who is backing RFKjr which is deeply troubling.

Spoutible is not open about its moderation methods or team, or its development team. It has had repeated software and security issues, which the owner tends to lie about, and then attack the critics, even making those attacks personal.

And Twitter is intentionally being destroyed as an organizing platform for one party, while being strengthened as a propaganda-spreading platform for the other party.

I should also add that I am writing long form articles (or trying to) on Substack. But, despite their addition of "Notes" (or whatever they call it), it still doesn't feel to me like a viable social media alternative. Though perhaps Post, with its article/comment distinction has similar issues?
Summary of comparison:

@thomasafine Andreesen Horowitz A16 is a financier or holds interest in both Twitter and Post.

(They could also have interest in BlueSky, no idea how it is funded)

@raynetoday
AFAIK BlueSky still gets funding from Twitter, so at least indirectly getting A16 money.
@thomasafine

@FeralRobots @thomasafine I can't figure out if A16 is playing the spread by putting cash in Substack (which includes microblogging platform Notes), Post, Twitter, and then this spillage into BlueSky.

More like they're too lazy to place a well-educated bet on one platform with a modicum of common sense marketing and operations savvy.

@thomasafine How do these two offerings then end up any different from a Main Stream Media (tm) outlet that posts every article with a comments section below?

I mean, that's been toxic for as long as news outlets have been on the web. Comment in response to broadcast has never worked out to be "conversation", has it?

@Johannab I think those are toxic in part because nobody ever took them seriously. There's not a sense of community or anything.

I find Post feels sort of like social media and Substack doesn't. Not completely sure why. But I only plan to use substack as a place to write long-form articles that arebn't suited to social media.

@thomasafine
Substack is arguably designed to NOT seem like social media, to the extent that it's meant to allow people to develop their own quasi-independent brand. They were trying to have the best of all worlds: parasocial interaction with creators, but individual brands. Notes is supposed to create the sense of community, but it's not fully baked yet.

& then there's the weird shit about the platform like no one actually having a password until they force one to be created.
@Johannab

@FeralRobots @Johannab
Yeah that was annoying until I figured out how to set a password.
@thomasafine
It feels like some too-clever executive who never remembers his own password insisting that they just use a forgot-password workflow for logins.
@Johannab
@FeralRobots @Johannab
A couple of years ago it was popular to write articles recommending exactly this as standard procedure. Kind of nuts.
@thomasafine
I feel fortunate to have missed those. I think they might have made my brain explode.
@Johannab
@thomasafine check out @trunksapp as a nice client for Mastodon.