Alonso snubbed famous foe Sir Lewis Hamilton, with whom he had such an intense feud when they were team-mates at McLaren for one season in 2007 that he quit the team.
The Aston Martin driver has instead opted for Schumacher, whom he beat to the top in 2005 and 2006 to win both of his world titles.
The 41-year-old admits it is “difficult” to choose his fiercest ever opponent but the Ferrari legend’s dominance in the mid-2000s ranks him above everyone else.
Alonso told Bang & Olufsen: “It is difficult to choose one because they were all great rivals and very talented drivers as well.
“When I came to this sport, Michael Schumacher was dominating the races and everything, probably that fight with him would still be the one I would choose.
“Fighting against Michael was very special, so I would say that.”
Alonso and Hamilton rekindled their rivalry last year after the crashed at the Belgian Grand Prix.
The Spaniard put the seven-time world champion on full-blast as he branded him “an idiot”.
The ex-Renault star also claimed the British great “only knows how to drive and start in first” in a petulant attack.
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But the Mercedes superstar cooled their rift by insisting he is trying to “be a better driver”.
The 38-year-old then joked on social media that he’d offer Alonso a signed cap as a peace offering.
Hamilton said: “It’s not a thing for me. It doesn’t change anything, it doesn’t really affect me in the slightest.
“With the cap it was just a little fun thing. It’s good to have fun sometimes with these things. For me, I am always trying to be a better driver.
“Also, I know there are a lot of young kids that are following me and it’s important the steps I try to take in my behaviour, so I am always trying to learn and be better.”