Bluesky: 1M+ downloads
Mastodon (official client): 1M+ downloads (there are multiple clients but most downloaded one is probably the official client)
Threads: 100M+ downloads
As a user of Mastodon and Threads, I feel like Threads has the most active user base and also it is possible to find more popular or official Twitter users on Threads as well. The biggest lacking features are trending topics and DMs. Also the video player features are annoyingly lacking, can't even pause a video on web player. If they can implement these features quickly I think it is a viable alternative to Twitter/X.
I recently got invitation for Bluesky and briefly tried it, but unlike Threads, I couldn't find anybody who I was following on Twitter. Also it was a bit hard to discover new people, so I just left.