1.
Di Stéfano to Real Madrid. It's hard to imagine a more impactful transfer not only in terms of the trophies won/goals scored, but also in terms of completely changing club's overall stature. Beckenbauer/Cruyff did a similar thing for Bayern and Ajax, but don't count in this one. Cristiano had made a comparable impact in terms of an actual in-game contribution, but I don't think that Real Madrid's stature significantly changed because of him (only in short-term maybe), it was arguably the biggest/greatest club of all-time and it still remains one.
2.
Maradona to Napoli. A huge bet — a mid-table club signing a mercurial talent for a world-record fee, but it couldn't have turned out any better. Napoli weren't able to capitalise on the success that Maradona brought to them, and that's why it's hard for me to put this transfer on par with Di Stéfano's, but it's pretty close. The individual impact that Maradona had on Napoli is possibly unmatched by anyone in history of the game.
As for some of the other transfers (mentioned and not):
- not entirely sure about van Basten, although the summer of 1987 surely goes in history as one of the most impactful transfer windows of all-time. To get both van Basten and Gullit in one window is cheating (and I'm not even mentioning Rijkaard who came a year later). Van Basten's injuries stopped him from making a Di Stéfano/Maradona-esque impact on AC Milan's history, although he was capable of reaching comparable heights
- Cristiano probably makes a top-5 — it was a huge, huge sum but they've pretty much got it back by selling him and his individual impact is simply insane
- Puskás is probably the greatest free transfer of all-time
- Cantona should be somewhere on those lists, although probably not in the top-5 considering lack of European success
I'd like to also mention those, even though they don't really compete for the GOAT transfer status:
- Gaetano Scirea. He came to Juve from Atalanta after 2 seasons of professional football and ended up winning 7 Serie A titles, 2 Italian Cups and literally every available UEFA Club competition — all while being the most integral part of one of the greatest defensive units in history. Considering his overall impact on Juve I think he can be considered their most important transfer of all-time, although he has some serious competition in Buffon, Charles/Sívori and a few others
- Javier Zanetti. It's impossible to imagine that Zanetti actually had a career before Inter, and that says it all
- Luigi Riva. He was signed by Cagliari as a 19-years old, for a considerable sum and he would spent the rest of his career there, leading them first to a promotion from Serie B and proceeding to win a Serie A title (it's arguably the most impressive underdog achievement in Serie A history, topping Maradona's Napoli and Elkjær's Verona), then coming close to winning them a second one: they were leading the table before he got his leg broken. He also refused a multiple transfer offers from Juventus that would've made him the world's most expensive player at the time. With 3 capocannoniere awards and a golden goal in the 1968 Euros final he's generally considered to be the greatest striker (number 9, so he's not competing with Baggio or Meazza, only with Piola) in Italian history
- László Kubala. Famously the man who had forced Barcelona to build Camp Nou as so many people came every week to watch him play. Also officially the greatest player in Barcelona's history — a title that sounds a bit ridiculous now, but the vote was taken in 1999, so it didn't include the now-obvious suspects.
There are a few transfers in United's history that also can be mentioned alongside those, most notably
Law,
Robson and
Keane.
Henry for Arsenal should also be taken in consideration when we're talking about the most impactful transfers in history of English football.
Lampard from West Ham was also a bargain and I'd say that he's probably the most consistent/best player in Chelsea's history.
I like this one as well, although, again, it probably doesn't deserve a spot in top-3 or top-5 considering his relatively short peak and the original strength of that Juve side. Still, he was a Cantona-esque catalyst that allowed a fantastic Juventus team to make that final step up and get their first ever European Cup — introducing them properly as a part of a European elite and not "just" as a domestic giant.
P.S. I forgot that Gerd Müller wasn't an academy product! He's for sure getting straight into the top bracket. Probably as a #3 or a #4.
