Mastodon has successfully replaced Twitter for me, so it's by far my favorite. It does still need better tools for dealing with large-scale posts and users, but overall it feels like it's actually doing the job I want done.
I want to like Piefed/Lemmy more than I actually do. The Fediverse answer to Reddit just doesn't feel ready for prime time yet. It's hard to find/connect with communities and the user base doesn't have that "can address basically any question" magic.
Hashtags have been pretty helpful to me on Mastodon. If people tag their stuff with the tags I follow I get a chance to see me stuff and follow new people organically, while people who didn't want to be discovered never hit my radar. Making that kind of topic tagging easier for people to apply might be useful, as I've seen new arrivals to the Fediverse who can't figure out why nobody sees them until someone tells them to tag their stuff.
The only reason I had a Twitter account was because there was an emergency event in my local area and Twitter was the one place I could get information about it right now. There were locals sharing what they knew, emergency services telling people what measures needed to be taken where, and journalists on the ground saying what they knew in real time. It was invaluable.
When I left Twitter, that ability to follow breaking news as it happens was the thing I was afraid I'd miss out on most. It's bittersweet to find out that I didn't need to worry about that after all.
Following hashtags is really powerful and useful on Mastodon. You can click any hashtag on a post to see other posts that use it, and if you like it there's a button you can click to start following that hashtag. You can also search for hashtags in the Explore section. Since there's no algorithm, hashtags are the primary way to get things that interest you into your feed.
@FediTips@social.growyourown.services posts tips on how to use Mastodon, so it's really helpful to follow as a newbie.
@FediFollows@social.growyourown.services posts lists of interesting accounts on Mastodon, usually by topic. It's a good way to grow your follow list!