The BEST anime...huh? Well there's a bunch of those that would fit into the category, based on opinions. My personal favorite right now would have to be Shingeki no Kyojin (Or known as Attack on Titan). I say this because there hasn't been an anime in a while that centers around the uncertainty of every character's survival every episode.
You're never sure what will happen next or who will fall to the Titans. Every episode you pretty much sit on the edge of your chair to see if the Titans are on the move again, what mankind is doing in order to protect itself and how the main character will cope with his uncanny power.
Give it a look, I personally think it's fantastic. Then again if we're going with classic animes I'd have to say Cowboy Bebop, Samurai Champloo, or Trigun. Those are pretty much some of the best ones out there. Oh and Sword art Online is great to, except for its season 2 kinda fell off a bit, but season 1 was fantastic. I've watched it multiple times now. And now i'm rambling on so i'll stop here. If you need more, don't be shy to ask.
~Link