I really liked Global Game Jam as it was a great chance to team up with people I hadn’t met before and learn new things. I love Ludum Dare as you get loads of feedback. I also have enjoyed js13kgames and #Nar8 gamejam.
I very much like the Familiar Game Jam (which happens in Spain but also online) all of the games that are made are always of such high quality. The quarterly GM48 is also interesting in terms of the games that are made in them. I feel like the GM48 also has a great community around it.
Covering the spectrum of game jams, I’d say…
Ludum Dare for 48h online jam
Global Game Jam for 48h in-person jam
1 Game a Month if you want to make release-worthy games
1 Hour Game Jam for quick bursts of creativity and skill refinement (I’m affiliated with this one)