My fondness will always belong to the Rath Cycle. Unforgettable to me, and actually, in some aspects, quiet defined modern Magic, and revolutionized it.
But, the last set that I've ever played (over MTGO) that I was a total "WOW" factor, and still has that effect to an extent, is Time Spiral.
Ravnica was fun on the "outset." But as soon as you play it and notice how caged the concept is, it grows boring very fast. Sure, a lot of awesome cards and great ideas. But it was too reinforced and enclosed with its color divisions. It didn't feel natural. Did not have a flow. And every time you played a deck with a lot of Ravnica card, it just couldn't be escaped that it felt like you're playing the same deck...
Yes, it's one of the most successful Magic sets ever, if not the most successful one in the game's history - but that's a financial success. Based on what? The array of interesting individual cards that existed. The allure of the concept. It was the one set that, upon release, I spent $300 for MTGO cards and boosters.
Few weeks later, I realized how boring the set was.