For me it was the 2016/17 UCL top scorer battle. After the round of 16 ended, Messi was the favourite to finish the season as the top scorer, with 11 goals in only 7 games played. At the same time Cavani had scored 8, Lewandowski and Aubameyang had scored 7. Meanwhile Ronaldo had scored only 2 goals until the quarter-finals started, even his teammate Morata had scored 3 and he only started one game in the competition.
Flash-forward to the end of the campaign and Ronaldo finished the season with 12 UCL goals, after his brace in the final to help Real Madrid in their 12th (then) UCL trophy. A truly iconic turn-around that isn’t talked about much in my opinion. Any other suggestions?
I’m biased as a City fan, obviously, but after Kompany and Lescott, we’d gone on a streak of signing defenders that could not cut the mustard at the level we wanted to play at. Demichelis, Otamendi, Mangala. Then we signed John Stones. He was okay at first, but then dropped off - from what I remember, it was a mixture of personal issues and injuries - and everyone including our own fanbase had him in the same bracket as the Mangala archetype, that being very talented but never living up to the potential we clearly signed him for. Even the club seemed to give up on him, signing up Aymeric Laporte to presumably fill the gap he was going to leave in the squad.
Over the next two seasons, though, he went from being the player that everyone wanted to be dropped, the player that we were absolutely certain was going to be sold just as soon as a club would take his large wages, to being a decent backup, to being on a good run of form, earning a new contract and some trust from Pep which then propelled his form to another level and he hasn’t let it drop since. He’s now an absolute hero to the City faithful, and his nickname of Johnny Boulders stuck. That seemed to give him all the confidence he needed and now there’s less confidence in the team in general when he’s not playing - certainly not as impactful as Rodri when he doesn’t play, but still significant. Hell, even Football Manager introduced their new “Libero” centre back role as the “John Stones role” - the player who starts at CB and steps into the midfield in possession to create a double pivot of DMs.
I’ve been watching football for a long time and I’ve seen a lot of players who signed for a club, didn’t live up to their expectations, and got sold on. I’ve seen arguably more of it at City because of the rapid build to success - Roque Santa Cruz, Robinho, Bony, Mangala, Jack Rodwell, Jesus Navas, Fernando, Claudio Bravo, just to name a few through the years. I don’t think I’ve seen someone get themselves into that hole and then dig themselves out of it to the point that they’re seen as absolutely critical to the team’s success as well as Stones has.