From my knowledge, most of the series seem to not have much to do with each other. Fate/Stay Night and Fate/Zero are considered the core series, with SN being the prequel. The main problem with SN is that the majority of it is only viewable as a visual novel instead of an actual anime.
If you want to get the main gist of the Fate storyline only in anime form, I've heard it's best to watch "Fate/Stay Night: Unlimited Blade Works", and then "Fate/Zero". There's a movie called "Fate/Stay Night: Heaven's Feel" coming out this year, which is supposed to be watched before Zero, but I didn't wait for it obviously.
If you want to get every bit of the main storyline, it's suggested to read the entire F/SN visual novel, then to follow it up with the F/Zero anime, UBW, then the HF movie whenever it comes out.
(As a side note, I myself watched Fate/Zero first without knowing what a big franchise it was from. In my opinion, it was perfectly fine as its own stand-alone anime, and UBW just fleshed things out with a bit more history and characterization imo.)
The rest of the titles are spin-offs that, like I said, don't contribute too terribly much to main story, if any at all.
Yeah, Fate is just really complicated.
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