what was the "best" league yet, in POE for you?

Harvest. No contest. Though both Legacy and Harbinger come really close. Honorable mentions go towards Perandus.

As mechanic, I loved Harvest with garden and seeds, for rewards Sentinel + Delv (just love delv).

+ for Blight and Legion.

Harvest

Blight- minion's power was amazing/ oils
Scourge- absolute amazing possibility for item's corruption/new tainted currency
Betrayal - a lot of new unique items/ veiled affixes
Legacy- play as you wish/ huge diversity
Ritual-large pool of players, even at the end of league/ good implementation of Harvest
Heist - replica items/new versions of gems/ impressive drop - ex for 35 c :))

Harvest for the gear progression
I miss the availability in crafting

Ambush was fun, especially because the game still had races
I wish they'd bring back race seasons

Archnemesis for the customizable challenge/reward system
The AN modifiers should never have gone core, though, IMO

Outside of the loot and AN mess of this league, I think that the base game with the beautiful Atlas Passive Tree and all the ways to get playing agency to do content you enjoy makes modern POE light years better than it used to be. So, I am not including any older leagues.

Regarding league mechanics I enjoyed playing the most - Scourge and Sentinal. Both of these mechanics made mapping fun and strategic (and Sentinal made Delve fun also). I also loved Delirium but Scourge had more strategy.

League mechanics I absolutely hated? Archnemesis recipe league. Horrible beyond description and the only league I ended up skipping the mechanic. I looked at the league list and the rest were fine (for me) all the way back to Bestiary (which sucked). Even this league mechanic would be fine if the rewards were better.

I used to ask this every few leagues or so, and the most popular answer was legacy.

Normally there would be a lot more mentions, but I'm guessing that's a reflection of a loss of interest and people moving on or simply taking a break.

Delirium was probably my "best" league given that it was my first 40/40 and I think it was one of the better "balanced" leagues as far as difficulty vs build potential. In comparison, right now it feels like you're pigeonholed into somewhere between 5-10 different core builds or a variant of them.

Harvest and Delirium are my favourites. I hated Blight, Synthesis, Metamorph and Legion. League mechanics that force me to play specific archetypes are the worst, such as Blight and Legion

Harvest! Although a got a mageblood in sentinel, i like the deterministic crafting!

Harvest. Played all league till the last minute.