I would recommend the Zenith Remmenant if you only want to buy one of the three, no question.
Children of Neinzul introduces a comparatively small number of ship types that all considered add little to the game; the neinzul fleet ships in particular are of little use to the player, make little appearance in games and function much more effectively in the hands of the AI, which uses them in vast swarms that are difficult to repell but self destruct after a few minutes. the music is v. inferior in comparison to that of the other expansions and I actually replaced all but one of the tracks with duplicates of the zenith remmenant music. the minor enclaves & enclave starships are excellent, and worth the purchase of the expansion if you want the complete AI war experience, but as a whole this one is the lowest priority.
the light of the spire adds a HUGE amount of content, sans amount of gameplay and ship types; the point to be wary of is that it is currently very much in beta development, incomplete, lacking music and some functions and generally rather imbalanced for now.
so yeah, imo: zenith remnant, Light of the spire, Children of Neinzul in that order.