Let’s look at the last 6 blocks: Odyssey, Onslaught, Mirrodin, Kamigawa, Ravnica, and Time Spiral. Is each block really better than the last? I think so. Odyssey had some flashback but Onslaught had the interesting bluff mechanic morph. Onslaught wins. Mirrodin has the lack of colors and the infamous Arcbound Ravager and Skullclamp. While Mirrodin had some downsides, I think it beats out Onslaught, but your mileage may vary.
Now, Mirrodin versus Kamigawa. Kamigawa featured soulshift and the cool dragon cycle that feature Keiga, the Tide Star, Kokusho, the Evening Star, and Yosei, the Morning Star who all affected constructed and limited. I still say Kamigawa beats out Mirrodin. Maybe I just like the artwork better, who knows. Kamigawa featured many cool legends as well as flip cards which were fun. Although Saviors of Kamigawa was a disappointment to all, it was bad even for the 3rd set of a block, it did feature the outstanding Pithing Needle.
Ravnica beats out Kamigawa for too many reasons, partly because Saviors of Kamigawa sucked and Ravnica had more colors than a tie dye shirt. Then Time Spiral came along, with its injection of nostalgia. Time Spiral itself was more than 400 cards, the biggest block yet, and introduced the purple rarity much to the confusion of collectors everywhere. One purple in every pack, but hope you didn’t get stuck with Squire. So Time Spiral, with its new subset of cards with cool frames, trumps Ravnica.
Will the next block beat out Time Spiral, maybe, maybe not. If Wizards of the Coast keeps working at it, they really could be getting better and better, and face it; it is in their best financial interest to keep improving.