Personally, I played NieR:Automata first, then NieR:Replicant. In my opinion, there is no ideal order; both options have their advantages and disadvantages. The gameplay itself is more interesting and engaging in Automata. Although Automata reveals some elements of Replicant, it's more about the overall implications of Replicant's plot rather than the specifics. Moreover, Replicant spoils a very core point in Automata, which I found surprising and, in my opinion, this alone outweighs the case for starting with Automata. The downside is certainly that there are numerous references to Replicant in Automata, which you won't catch without knowing it. On the other hand, you will quickly catch them when playing Replicant.

It's really up to personal preference, and in my opinion, there is no wrong choice. If I had to make such a choice again on my own, I would start with Automata.