Van Basten & Gullit in one summer would be my personal favourite (and Ancelotti as a bonus). Van Basten at his best was not far off peak Cristiano although his injury record lets him down & Gullit was on another level to Benzema (if both Cristiano & Kaká would've succeeded it probably would've been comparable). Although the longevity of Cristiano and especially Benzema is a bonus, of course.
Madrid's 2009 window was insane — Cristiano, Benzema as well as Xabi Alonso (he probably doesn't make it in Madrid's All-Time XI ahead of Redondo/Modrić/Pirri/Kroos but he has to be close). Signing Kaká for $75 mil has to be mentioned though, there have been many more expensive flops since then but it turned out to be an extremely poor transfer for them, considering the expectations & the price. But even with the inclusion of Kaká it's still like 9,5/10 window.
Chelsea in 04/05 have managed to completely transform the team by signing Drogba, Carvalho, Cech (as well as Robben, Ferreira, Tiago & Alex) — and, most importantly, Mourinho. We don't tend to include managers in those transfer assessments often but I can't ignore him. The fact that Chelsea went on to have one of, if not
the best seasons by any team in Premier League history shows just how incredibly spot on their transfers were.
A slightly less impressive but one of the best transfer windows by any club has to be
City's 2010 with Silva, Touré, Milner, Dzeko and even Balotelli. If Agüero would've came a year earlier it would've been a no-brainer.
Inter from 2009 is the very definition of short-term success & market shrewdness. To sign Milito, Eto'o, Sneijder, Lúcio, Motta & Pandev and finish the window with a positive transfer balance
A few of the more obscure ones:
Hellas Verona in 1984 had signed Hans-Peter Briegel and Preben Elkjær-Larsen that would lead them to one of the unlikeliest league titles of all-time. Larsen would finish 3rd, 2nd and 4th in 1984-1986 Ballon d'Or votes respectively.
Juventus in 1982 had decided to sell their Irish star Liam Brady to clear the space for 2 incoming foreign signings (which was the limit in Italy at the time), Michel Platini & Zbigniew Boniek — the pair will lead them to Juve's first European Cup (Boniek had dived for a penalty chasing Platini's trademark through-ball, Platini had calmly slotted it home). They were the only transfers of that summer, the very definition of quality over quantity. Platini is probably the greatest Juventus ever (even though the list of their stars is quite extensive — Sívori, Platini, Zidane, Del Piero, Nedved). Speaking of Nedved, that
2001 window by Juve was masterful — selling your best player & striker for a combined fee of $125 mil was supposed to hurt them... but they've signed Nedved, Buffon & Thuram
Inter's 1988 window was obviously overshadowed by AC Milan's Dutch trio but it was a masterful move to close the distance between them & their city rivals. Peak Lothar Matthäus & Andy Brehme (a pair that would win Inter the league title, beating both Dutch Milan & Maradona's Napoli only to follow that with the 1990 World Cup win with West Germany) for a combined fee little south of $4mil... Bianchi, Berti & Diaz that were signed in the same window played a big part in that league success as well.