Good question! DLCs contain variations of blocks in different themes, and some contain functional variants that combine existing functionality differently. What do you enjoy building, bases, tanks, spacecraft, factories, vehicles? https://spaceengineers.fandom.com/wiki/Category:Downloadable_Content_Blocks

Downloadable Content BlocksSpace Engineers Wiki

Look at the previews on Steam, there will be some DLCs where you feel you "must have that" and others where you don't care at all. E.g. I love the neon scifi blocks! ;-) Some people love the new shapes of the Heavy Industry or Warfare designs, etc. And some DLCs only contain interior furniture, if you enjoy building cozy stations, you will want these, but if you build tanks, you won't care about a kitchen or jukebox. XD Some contain suits skins, or armor skins, or emotes -- is that important to you? These are criteria that I would use to choose. Looking forward to hearing what you picked :)