Jagged earth has the most stuff, so maybe get that one first? But yeah, they all add good things

The order of their release is also approximately the order of increasing complexity, so maybe get them one at a time. The exception is Horizons which is the less complex spirits so maybe start with that especially if you're still in the "teaching to friends" phase

If you want incremental changes, branch and claw, then jagged earth, then nature's incarnate. There may be another small expansion at the same time as branch and claw.

If you are hungry for anything, horizons has 4 new spirits that are easy and fun to play. (If you have only horizons, check the base game, it rocks)

Either Branch and Claw or Jagged Earth next. Both add an essential mechanic (events) with Jagged Earth having more content.

After you get either, the order of the rest does not matter too much, though if your group would prefer more simpler/straightforward spirits, the standalone game Horizons of Spirit island contains good, simple to play (but fun!) spirits that can be used in the main game (and also provides a slightly simpler version of the game to introduce new players with).

Personally I'd suggest Jagged Earth > Branch and Claw > Feather and Flame > Nature Incarnate, and Horizons if you want simpler spirits or run out of other content to buy and need more.