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Boca Juniors - Roman Riquelme

Villarreal - Roman Riquelme

Liverpool - either Kenny Daglish or Steven Gerrard or John Barnes or Kevin Keegan?

Barcelona - Lionel Messi?

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Bayern Munich - either Franz Beckenbauer or Gerd Müller?

Italy
AC Milan: Baresi/Maldini
Inter Milan: Bergomi/Ronaldo
Juventus: Del Piero/Baggio
Lazio: Nesta/Verón
Napoli: Maradona

Spain
Real Madrid: Zidane/Cristiano
Barcelona: Messi/Ronaldinho
Valencia: Aimar/Villa
Atlético Madrid: Sánchez/Torres
Villarreal: Riquelme

England
Manchester United: Cantona/Best
Liverpool: Torres/Gerrard
Arsenal: Henry/Vieira
Chelsea: Lampard/Zola

Germany
Bundesliga: Beckenbauer

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Chelsea - either Frank Lampard or Petr Cech or Didier Drogba or John Terry or Gianfranco Zola?

AC Milan - Baresi/Maldini
Inter - Meazza/Facchetti
Juve - Scirea/Del Piero
Roma - Totti/Di Bartolomei
Napoli- Maradona
Real Madrid - Di Stefano/C Ronaldo
Barcelona - Messi/Kubala
Liverpool - Dalglish/Rush
Man Utd - Charlton/Lingard
Arsenal - Henry/Bergkamp
Chelsea - Drogba/Lampard
Man City - Aguero/Silva
Bayern - Beckenbauer/Gerd Muller

For Chelsea it's either Lampard or Zola, depending on who you ask. I'd personally give it to Lampard, due to longevity.

Terry, Cech, Drogba and Hazard are in the conversation, but I think Lampard did more, for longer.

Milan - Rivera/ Maldini
Inter - Meazza/ Facchetti
Juventus - Platini
Roma - Totti
Napoli- Maradona
Real Madrid - Di Stefano
Barcelona - Messi
Liverpool - Dalglish
Manchester United - Best
Arsenal - Henry  
Chelsea -  Lampard
Manchester City - Aguero
Bayern Munchen - Beckenbauer

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Ajax Amsterdam - Johan Cruyff

Flamengo - Zico

Real Madrid's no.1 is Di Stefano without question tbh

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PSV Eindhoven - either Ruud Van Nistelrooy or Willy van der Kuijlen?

Adelaide United is hotly debated between Eugene Galekovic and Isaias but I'd personally go with Isaias.

Sí Señor

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Feyenoord - Willem van Hanegem

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Santos - Pele

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Paris Saint Germain - either Zlatan Ibrahimovic or Pedro Pauleta or Rai or Safet Susic?

A bit surprised about De Bartolomei's mention by M99
Surely a good player , very loved from his people  but not a top player imo  ( 0 caps with Italy ) and  especially in that period  ( that era who gave 2nd Scudetto and CL final ) Roma had top players as Bruno Conti and Falcao.
Well, Totti is best Roma player  ever and that seems obvious . Anyway  I rate  Bruno Conti  on 2nd place.
Not just legend of  the club ( as Ago on the other hand) , but also amazing player of international level ,  I mean the best Italy player in World Cup 82 . Falcao might complete the podium, one of stars of that Brazil under Santana  too

About their cousins , Chinaglia and Nesta are best Lazio players ever imo. but tbf Lazio had also Piola , the top Serie A scorer ever

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Corinthians - Rivellino

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Another thing I feel sure is that Michel Platini is the best Juve player.
I wonder what Juve player think it but for me it's so.

Olympique Marseille ? I' d say Jean-Pierre Papin.

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Valencia - either David Villa or Mario Kempes?

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Deportivo La Coruna - either Bebeto or Juan Valeron or Djalminha or Diego Tristan or Roy Makaay?

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Blackburn - Alan Shearer

Milan - Maldini/Baresi
Inter - Meazza/ Facchetti
Juventus - Platini/ Del Piero
Roma - Totti
Napoli- Maradona
Real Madrid - Di Stefano/Cristiano
Barcelona - Messi/Xavi
Liverpool - Dalglish/Gerrard
Manchester United - Raiola/Mendes
Arsenal - Henry/Bergkamp
Chelsea -  Lampard/Terry
Manchester City - Aguero/Silva/Kompany
Bayern Munchen - Gerd Muller
PSG - Neymar/Mpabbe/Di Maria/Verratti/Gueye/Paredes/Meunier/Bernat/Silva/Marquinhos
Spurs - Lineker/Gascoigne
Dortmund - Reus/Weidenfeller
Ajax - Cruyf/Van Basten
Benfica - Eusibio/Nene
Lyon - Juninho
Marseille - Papin/Deschamps
Grimsby - Unique/Mole's uncle
GL - RO/Dani
USA - Columbus/UK
Saudi - UAE/Israel
Covid - Wuhan/FBI
Hagia Sophia - Erdogan

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AS Monaco - either Victor Ikpeba or Ludovic Giuly or Claude Puel?

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Real Zaragoza - either Diego Milito or Carlos Lapetra?

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Borussia Dortmund - Matthias Sammer

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Galatasaray - either Gheorghe Hagi or Metin Oktay?