The Fate franchise is now omnipresent in the anime world, having grown from a visual novel into a genuine multimedia empire. Fate/Zero is a good starting point for (adult) newcomers as the earliest chronological point in the primary storyline, but most importantly, gripping in its own right. The premise is nearly the same as every other in the Fate series: a battle for the Holy Grail. Heroes from the past are summoned and commanded by mages as warriors of different classes, depending on the attributes their legends granted them, and compete to the death. What makes this entry stand out is the dark, gritty tone, which comes from its focus on the sacrifice and loss inherent to the life of a hero, going as far as wondering if the Holy Grail Wars are all that worthwhile. The excellent production values and a few exciting confrontations are good vehicles for its overarching themes, so if you’re at all interested in getting into the franchise, this is likely the best starting point.
When you’re done with that, try: Fate/Stay Night Unlimited Blade Works, Psycho-Pass, Madoka Magica