If you only play one NieR game, Automata is the one to play. If you liked it enough to want a second helping, Replicant is probably the best form you can experience that in. It still feels a little more like medicine than dessert.
I wanted people from my group to also play the game, to the point where I promised them a free game of equal value if they picked up Nier:Automata, played through the whole thing, and said they didn’t feel it was worth full price. I REALLY wanted them to play it.