Strictly speaking about the anime?

Fourth & Fifth Holy Grail War - the “Main Story”

Fate/Stay Night - takes place in 2004 during the Fifth War but there’s three alternate versions of the events (Fate - FSN06 Unlimited Blade Works - FSN:UBW Heaven’s Feel - the movie trilogy) It’s a bit like What If suggested order is what I wrote but I HIGHLY recommend Heaven’s Feel AFTER at least watching one route and Zero.

Fate/Zero - the prequel that takes place in the 90s during the Fourth War.

The shows like Apocrypha, Extra, and all the others are in alternate timelines/universes and don’t really have a watch order. If I explain those I’d be here all day. Here’s a flowchart for simplicity’s sake.