I'd read Anna Karenina before. I enjoyed it then. I loved it this time. The story is good and the characters complex. But the true excellence is in Tolstoy's writing. There are times when he captures a moment or a sensation so perfectly that it is like the crescendo of a symphony or stumbling across a marvellous vista.
Yes, he takes something like three chapters to describe how Levin joins his peasants to mow his fields, but every line of it is engrossing. The simplicity and beauty of the scene is captured perfectly. Or the moment when Karenin forgives Anna, which is perhaps one of the most compellingly Christian yet complex moments in literature.
Likewise, as a symphony falls into the most mournful and melodious of tunes, you feel as much as read all the tortuous longing and disappointment of Anna, Vronsky, and Karenin. And, when Kitty gives birth, the anxiety, joy, confusion, and shared pain between her and Levin is almost visceral (while somehow you still find yourself chuckling at Levin's befuddlement).
It is, in a word: brilliant.