Quick Disclaimer: Spoilers for Fate Zero in this review. If you haven't seen that, go watch it before you start Unlimited Blade Works.
Unlimited Blade Works stands in the less than enviable position of being the first Fate/stay night adaptation following the small dumpster fire that was the orriginal fate/stay night. And yet despite that, UBW stands tall not only as a worthy adaptation of the story line, but a mighty peak of the franchise as a whole.
Every single aspect has improved since the first Stay Night. The characters are far more interesting and nuanced, the action is powerful, the animation is wonderous, and the plotline is supreme. UBW gives viewers who only watched Zero a wild ride, while also being enough of a different story to Stay Night so that those who watched the orriginal still feel engaged.
UBW is the path of the VN that focus on Rin Tohsaka, so obviously she gets a much more focused role than she did in Stay Night, allowing us to fully understand the tragedies she's gone through, and the strength that she's capable of. If one complaint could be lodged against UBW, it's that the characters focused on in the other two paths are less developed here to make up for it. That's not a big deal for Sakura, the focus of the Heaven's Feel series, but for Saber, who was Stay Night's focus, it feels like she's been robbed of a full arc. Stay Night never did her justice, and while UBW gives her a few moments in the spotlight, she's still mostly on the sidelines.
But this is a very minor complaint for a series as good as this one. With a powerful soundtrack backing animation that I must once again remind you is stellar, UBW is the best of the Fate series, and the bit everyone should watch at the very least.