You can play any game as a standalone game, but if you're engrossed in the series you'll get more out of each game. Drakengard 3 in particular, and Automata to a (much) lesser extend.
points out, Drakengard isn't particularly enticing in the gameplay department. With the benefit video playthroughs, you can watch a cut of the games cutscenes (with relevant gameplay bits) on youtube:https://www.youtube.com/watch?v=EkmtlWhn_20
However, the gameplay and its tedium is very much by design. If you want to fully grasp the experience of the game, I recommend you at least sit through a single encounter.
Nier departs, though not much. Drakengard 3 goes back to its roots but takes some hints from Nier. Automata's gameplay was provided by a different studio and so is the most different.
Nier 2 was made by a different team without (direct) involvement by Yoko Taro, and this is evident. You can safely cross this one out; it's entirely optional and provides almost no additional context.
By the way, my personal bracketing for these games is:
Drakengard 3 > Drakengard & Automata > Nier
All of them are good, and all of them are divisive even among Taro fans. But, man, if Drakengard 3 hits you, it hits like a freight train.