So very, very close, but... (spoilers in comments)
The first 24 episodes of Fate/Zero
play out like a master class in how to write an ensemble cast. Most of the main characters are given enough depth and complexity that any one of them would be enough to carry a different show by themselves. Put together, the clash of each of their distinct personalities, goals, and philosophies forms one of the most compelling shows in recent memory. Special mention must go to Rider, whose philosophy of kingship comes to define the show's B plot, and Kiritsugu, who is essentially a walking deconstruction
of the very concept of heroism.
The show also excels on the technical side, with Ufotable bringing some of the highest production values in TV anime history to the table. The Japanese voice cast is pitch-perfect, with Joji Nakata in particular earning points for able to seamlessly switch between the different facets of Kirei's personality. The English dub is a bit more inconsistent - while most of the performances are just as emotive and effective as in the Japanese, it suffers from a rather severe case of Lip Lock
that results in lots of extremely... awkward and immersion... breaking pauses in the... dialogue. If that doesn't bother you, then go for it.
Unfortunately, the show's ending is a huge disappointment. The problem is not with the actual events, which mostly make complete sense when written down, but with the execution. In brief: This is a prequel to another story, so its ending was set quite firmly and very specifically in stone from the beginning. And yet somehow, it still
managed to come feel like Urobuchi pulled it out of his ass at the last second.
I've played all three routes of Fate/stay night
and even I had to rewatch the last episode in an attempt to figure out just what had happened. When that didn't work, I had to look up the original light novels, which were only mildly helpful.
I think what happened is that, because the author assumed that the target audience would already know how the story would end, he forgot to include any kind of foreshadowing that would allow the work to stand on its own. If the concepts and mechanics that resulted in the ending being what it was were explained - or even hinted at - earlier in the show, it would have flowed far better. But as it is, the ending is a stain on what I would otherwise unhesitatingly call an instant classic.