Ravnica, by far.
I started collecting as a little kid (age 7 when 7th edition came out), and stopped soon-after because no one else my age was playing. Now, I kept up with the game a bit, but that was mostly reading library copies of Scrye in the Mirroden/Kamigawa days. By the time Ravnica hit, I had only caught brief glimpses, and probably didn't care.
But at some point, I went back. This was by the time we had shit like Gatherer. And I poured over the entire block. Then I saw my library carried a bunch of MtG books. To this date, the Ravnica books are the only series I'd finished (kinda fell off the Kamigawa and Lorwyn books.)
And just looking back on it, so much of it was just so right.
>The storyline of the individual sets was invisible, but looking at the current blocks I'd say it's a plus.
It was too busy establishing the world to bring any individual story into it. There were guild members making snide (sometimes poorly written, sometimes clever) remarks about other guilds. The constant talk of how damaged the environment had become, to the point where you didn't know if some of the cards were magic or just things that happened (Caustic Rain, Rain of Fire, Seismic Spike).
>They even gracefully got away with the need to follow up with more spirits after Kamigawa, by suggesting there was some force keeping them there. It really didn't matter what it was, just that there was room to play with it.
>The flavor just oosed together to form a crapsack world that almost seemed functional. And all the guilds had a very novel approach to their color combination.
>Even the moderate lack of citizen cards was fitting: the common man simply isn't important.
Sure, a guild might bribe a criminal, or push you into the wrong alleyway, but for the most part, you had to have some sort of job (guild or not) or specialized skill to show up. It was all dog-eat-dog and every man for himself. Otherwise, you were another face in the crowd at best, and at worst a casualty.
>Then there's the art. I always loved the old art style, and adding cityscapes to it was just gravy.
>Even the power level wasn't obscene, but we didn't have Mythics or Planeswalkers back then. But that's another rant.
>Finally, there were a lot of really unique cards.
Yes, a lot of sets had stuff that pushed the envelope on design, but some of the things that really stood out are the Nephilim (especially Ink-Treader), the nonstandard Simic creatures (Omnibian, Cytoplast Manipulator, Novijen Sages, Project Kraj), and some of the Azorius spells (Dovescape, Palladian Accord). There's more, but I don't have the time to look.
This isn't the first time I've gushed over the block, and it won't be the last.
(Pic related, a surprisingly well thought-out bit of flavor text.)