Just watch Fate/Zero, the director for Fate/stay night is terrible and butchered the already amateurish script to new heights. Fate/stay night is really only watchable if you're a fan of the VN 'cause otherwise the writing makes absolutely no sense, especially the protagonist's actions. The main character in the anime doesn't even resemble the original character and given how the story focuses on him alone butchering his characterization effectively kills any enjoyment to be found in the anime.
So yeah watch Fate/Zero, skip Fate/stay night.
I've seen all animated works involving Fate/things, but haven't read/played the VN or anything else relating to the series beside the animated works and some wiki. I've found all entries to be easily watchable, even enjoyable. Even the first F/SN anime didn't feel like I had wasted my time with it. I also didn't feel like I didn't understand the stuff that was going on, beside a few Grail related issues that I still haven't looked up. Knowing how Shiro's brain works (or doesn't) does help understanding it (I read about it somewhere before the release of the UBW movie), but not absolutely positively necessary.
So yeah, F/SN was ok. F/Zero is brilliant. F/SN UBW has been somewhere in between those two. Heck, If you like well animated fight scenes, UBW is worth watching based on those alone.
I dunno, I've seen plenty of genuinely crappy anime and there seems to be a smattering of harsh opinions here on stuff that are actually just below average, average or even above average. It's like the scale is missing the middle tier altogether. But I guess this is a thing about how opinions are voiced.
