There's four that stand out to me. In no particular order, they are...
1. Triple Khans of Tarkir
Both morphs and wedges were really well integrated into this format. Tons of fun to draft, with a lot of guesswork (what does your opponent have...?)
2. Triple Rise of the Eldrazi
This format was quite genius, and really, really different from usual retail draft formats. It's way slower and grindier than most, and one of the really fun things is that the weirdest cards turn out to be playable in the format. Stuff like
Raid Bombardment,
Vent Sentinel,
Mnemonic Wall, and
Totem Hartebeest. It's probably the format where
Glory Seeker is at its worst (it's funny that I just lauded 3x KtK, which has to be the format where Glory Seeker is at its best). Anyway, speaking about formats with weird playable cars...
3. Triple Innistrad
This format is gas. Even after weeks of drafting it, pros were finding new strategies!
Burning Vengeance and
Spider Spawning have to be the sweetest build arounds ever to be viable in a draft format. A format that had really good aggro decks no less! Intricate, balanced, super sweet.
4. The final one is Kaladesh, which I found fun both on its own and with Aether Revolt. This block had so much going on in limited, and I played my coolest draft match ever with this set. I was doing side events at GP Rotterdam a few years ago, and made the finals with a sweet WB value deck running maindeck
Panharmonicon. My finals opponent copied it with
Saheeli's Artistry, and went to town with a
Whirler Virtuoso and an
Era of Innovation. Except, I had an overwhelming board position, so he had to keep chumping with his thopters, never amassing enough to profitably attack me, until at long last he found a key removal spell and I succumbed to a thousand paper cuts over the next few turns. When the dust settled, my ride, who said goodbye at the start of the round to go watch the top 8 draft, checked in on me to ask if I was ready to go. Turns out the t8 completed the draft portion of the draft, including deck construction, and were just about to start their first matches. Because these side events have no time limit, we had slogged it out for one and a half ours, and barely finished game one! Needless to say we decided to split the money (slightly in his favor, he did win game one after all), because after that epic game one, either game two was going to severely disappoint, or finishing the game would take another 1.5 hours, and we didn't feel either was a desirable outcome