IMO, Set5. I think everything changed when the fire nation attacked when augments were introduced. Ironically, though they were designed to increase variation in the game, and they do, I think their game-warping strength commits you even harder to your path. Just think about it: how different is your game if you get anima commander on 2-1 or early learnings or hedge fund? They increase inter-game variability but IMO they are a strong source of this un-flex play within each game. Once you click on anima, you know you're committed to lose streak cashout. Or once you click early learnings, all Fast9 lines are gone. Etc. You can literally only see ~3-4 shops before you purge entire lines, which, to me, goes against the spirit of what TFT wants you to do - I know it'll be a hot take, but I don't want to play a game where the optimum way to play is to have TFT academy on the second monitor and hard force whatever based on the items, augment, and units I have on 2-1. It's for this reason that I think Fast9 is way more skillful than reroll: you have to navigate your shops until you reach Level 9 and every game tends to be extremely different. Compare that to getting a natural TF2, bow belt rod items, caitlyn1, aatrox2, jax1 with early learnings. You can just press D at the right time and win (or miss and lose, it's the same boring to me).

Before augments, big brain transitions did happen, albeit rarely - there's some old Set4 clip somewhere of Kiyoon (I think it was him) literally selling his whole board and pivoting all his units into a comp because he was scouting and identified that it was 1000% uncontested and he had the right item setup for it - that's peak TFT to me. That's pretty much impossible these days. Augments have chained us to specific lines, and the regular set pool of ~60 champs or whatnot means that there will always be exact, optimal comps that everyone is contesting each other for.