Given its narrative structure in its original visual novel form, it's no wonder a pure adaptation of Fate/Stay Night is impossible. It's constructed with three different storylines, or routes, that must be played through in order. They're separate storylines, and yet they build upon each other as if they were part of the same narrative. They'll even make references to story beats and elements from the other two. The plot device of parallel universes isn't brought in until the third and final route, Heaven's Feel, but once that falls into place, the idea that these are all canon timelines existing simultaneously makes more sense, oddly enough. More so, when taken into consideration the fact that Fate/Zero, as a prequel, takes into account setting up all three of the Fate/Stay Night routes.
With that being the case, which route's adaptation did the best that it possibly could have in adapting it? Which anime adaptation of Fate/Stay Night did the best job of bringing the route it was adapting to life in animation, with a passionate narrative that best conveyed its story not just to fans of the visual novel, but also to anime-only watchers?
Deen/Stay Night
For a long time, the only anime adaptation that existed was the 2006 adaptation by studio Deen. Technically, it adapted the Fate route, which features Saber as the main heroine. However, the route itself was too bare-bones for a full series adaptation. Moreover, the creators appeared to have set forth on the premise that they could do like many other visual novel adaptations tend to do, and focus on one route but still pull the best parts from other routes and include them in the same adaptation, as a means to tie disparate storylines into a unified narrative, not unlike what series such as Clannad did. For what they came up with, it was an admirable effort. Anyone familiar with the visual novel could easily pick out the parts that were from the Unlimited Bladeworks route, and for the most part, those parts worked well enough alongside the Fate storyline, with enough of the former there for fans of the visual novel, as well as for anime-onlys, to be intrigued and make a fairly decent guess at things like how Archer's identity is tied to Shirou.
However, incorporating parts of the Heaven's Feel route proved tricky. That one has quite a lot going on, and can be confusing without being given breathing room to explain itself and its place in the story. That and Heaven's Feel's more explicit content meant that many of its story beats had to be cut out simply for their explicitness. In the end, Deen came up with an anime-original homage of sorts to the Heaven's Feel route with mixed results. Then they ended the anime proper with an original ending based on the Fate route. Given what they had to work with, it was a noble effort. Unfortunately, it resulted in the overall story being a bit of a mess in terms of story focus and pacing in some parts, even if it did find a way to give Tohsaka and Sakura their due even though this wasn't for either of their routes.
ufotable Takes the Wheel
For those fans who were upset that the Unlimited Bladeworks route just didn't get its own adaptation, Deen later came out with one, only just as a feature-length film. This, again, meant stripping the story down to its basics to keep the runtime down. So, while many were content at least that an anime adaptation of the Unlimited Bladeworks route existed at all, since for the longest time there was no discussion of any further adaptations, it's fair to say that visual novel fans of the film over the TV series were probably settling, deep down.
Then Fate/Zero was adapted into an anime, this time by ufotable, and it was a massive success. It reinvigorated the Fate franchise, even brought in new fans. And many praised it as being superior to Stay Night, which, comparatively speaking, wasn't exactly hard to be. Still, ufotable shot for the moon in terms of their adaptation of the orignal light novels penned by Gen Urobuchi. However, there didn't appear to be any talk of another Stay Night adaptation--that is until it was announced officially that they really would be helming a Stay Night reboot.
Fans were all the more excited that they were planning to do Unlimited Bladeworks. Likely the only ones who were not entirely happy perhaps were fans of the Heaven's Feel route. Thankfully, ufotable had that covered too, promising that they were going to adapt Heaven's Feel too, though as a film trilogy. Again, given its explicit content, a TV series was still out of the question, but ufotable had built their success on movie adaptations of Type-Moon properties from their Kara no Kyoukai series. So Unlimited Bladeworks and Heaven's Feel were both in good hands.
Conceivably, Unlimited Bladeworks has the best adaptation, simply because it has the most time to flesh out its story without having to shove in other route story beats to fill out anything for a 26-ish episode run. But while both of the ufotable adpatations are stellar to say the least compared to Deen's Fate route adaptation, there's something to be said with parts of the Unlimited Bladeworks route from the Deen adaptation played on repeat in the ufotable adaptation that could make some arcs slightly more tedious to get through, if only slightly.
While Heaven's Feel fans didn't get a whole series, most of the filler slice-of-life chapters from that route in the visual novel could actually be cut without losing the crux of the story. This meant there were few places where the pacing dragged, but the overall narrative remained coherent. And as the final chapter in Fate/Stay Night's story, it wrapped things up with the same satisfaction that the route itself gives its fans, as the darkest timeline with the happiest and most satisfying ending for most of its characters.