The very concept upon which the show is based - spirits of heroes from popular legends and myths coming forth to fight in a modern war which makes monsters out of normal people as they kill each other to get their wish granted by the Holy Grail - is fascinating enough in itself to compel one to watch the show. The great thing about Fate/Zero is that it uses all 14 participants (7 magi and their servants) in this battle royale for the grail to their fullest potential, with each character significantly contributing to the plot, as opposed to its sequel, Fate/Stay Night, which focuses primarily on a single character, thus wasting the potential of all other participants.
Although it is a prequel to Fate/Stay Night, one might as well consider Fate/Zero as a standalone work rather than a prequel, as it doesn't try so hard to play out the stories of Kiritsugu Emiya and company to set things up for Fate/Stay Night or to exactly match how things were in F/SN, but rather develops compelling stories for each of the characters.
Each and every character in Fate/Zero is used to their fullest potential in terms of their contribution to the plot, and each character, human, homunculus, or heroic spirit, is explored in depth. And that, I feel, is the show's strongest point. While war makes a monster out of many of the characters (although a few are arguably "monsters" to begin with), Fate/Zero dissects every single character, so that audiences are compelled to sympathize with the characters that would normally seem despicable, just because they develop such a deep understanding of them. Moreover, there are no black and white "good guys" or "bad guys." Although Kiritsugu Emiya is the main protagonist of the story, he is very morally grey. You will feel compelled to back various characters in the war, not just Kiritsugu.
On the topic of the protagonist, it's also important to note that another one of Fate/Zero's wonderful qualities is that the narrative doesn't center around the protagonist. Although Kiritsugu Emiya's back story is given more screen time than those of others, the Holy Grail War itself is the center of the plot, rather than Kiritsugu, and because of this, every participant in the war is [almost] equally represented. Audiences learn enough about the back stories and motives of each character to back anyone, and no character is guaranteed victory in the war either, as anyone can be killed, because there is no clear "hero" or "good guy."
Regarding the aesthetics, the character designs, especially for the heroic spirits, are absolutely gorgeous. The use of lighting and colours in various sequences is spectacular. Spectacularly animated, the visuals are backed by a soundtrack orchestrated not only to reflect the grand proportions and godlike beauty and magnificence of the Heroic Spirits, but also to capture the darkness of the war being waged.
To wrap things up...
The show has wonderful pacing, so that characters' intricacies are explored without bogging down the on-going plot. Nothing is black and white. Your feelings will be muddled and often times you will be confused about who to support in the war or how to feel about a certain character. It is a show that will force you to think about morality, honour, selflessness, and the so-called greater good, and how these values apply in a war that is essentially fueled by greed.
All in all, a masterpiece. 10/10. You will not be disappointed!
