Hot take: Whether to read the visual novel first or watch Fate/Zero first depends on your priorities.

[Uchiha_Madara]

Everyone wants to tell newcomers how they should enjoy the franchise, but I think instead of asserting opinions it is more helpful to lay out the facts and let each person decide for themselves what order to do this in. Personally I watched Fate/Zero first, but that was right on the heels of the series being completed and before I had read a single visual novel. I initially skipped over the 2006 Stay Night anime because the premise hadn't really caught my attention.

To the point of this thread, you have to ask yourself which is more important to you: Do you care more about watching the events in chronological order, or would you rather view them the intended way, in the order in which they were written? No matter which direction you chose to go, you are going to get plot points from the other side of the equation revealed to you early.

The original Fate/Stay Night visual novel was written first, and the Fate Zero light novel (which the anime adapted) started releasing in 2006, which is the same year that the original visual novel got its first anime adaptation by studio Deen. The story of Stay Night takes place during the Fifth Holy Grail War, while Fate/Zero takes place during the Fourth Holy Grail War, and during the events of Stay Night you'll come across details of what transpired during the fourth war, including who won it, who some of the participants were, and most importantly, how it ended. You are inserted into a world where the Fourth Holy Grail War has already occurred, and it has left its mark on the city of Fuyuki and its inhabitants, and in particular Emiya Shirou, the character through whose eyes you see the fifth war.

The story of Fate/Zero was written around the details that were revealed in Stay Night, with most of the details being filled in by Gen Urobuchi, Fate/Zero's author. Fate/Zero reveals ahead of time certain plot points that the visual novel assumes are a mystery to the reader, because it purposely peppers details about the previous war throughout the three routes, over time giving the reader answers as to why certain things happen and why certain characters behave the way that they do.

So again, which is more important to you, watching the events unfold in chronological order, or the order in which they were released? There's no rule saying that you must go through the events in the intended order, but by that same token it is the intended order, regardless of what some might argue over minor references or which spoilers are better or worse. Either way, you're not going to avoid spoilers no matter which route you choose to go, so you have a decision to make. It should also be noted that Fate/Zero as a whole has a much darker tone to it than the majority of Fate/Stay Night, so your viewing order could also impact your opinion of whichever work you watch second.