Most Memorable? I mean:
All joking aside.
The original VN, is one of the best VNs ever made, although not a true kamige, it saved Type-Moon and put it on the map (back in the day). It did what Tsukihime didn't.
The biggest appeal of Fate, is for us history nerds, just to see what they do with historical characters, although the theme of the series, is pretty much contemplation of what it means to be a hero.
IMO, not taking into account the og VN, peak Fate, is Fate Zero, Why? Kiritsugu, yeah that's it, although I have a big soft spot for Hollow Ataraxia as well (cause my boy Avenger is fucking awesome), but that one is never getting an anime anyway.
Whatever one you wanna go with, is fine, BUT, stay as far away from Grand Order as humanly possible.