Korchnoi and Fischer played each other ten times between 1962 and 1970 and are perfectly even +2=4-2 in classcial chess and one win each in blitz (this picture is from the notorious Curaçao Candidates' in 1962)

Viktor Korchnoi is undoubtedly the player with the most games and wins against grandmaster calibres in professional chess history.

Inluding simultaneous exhibitions all over the World, Korchnoi played maybe 20'000 games, more than 5'000 games in official events.

Korchnoi faced and beat great players from at least five chess generations (see more details below): from Levenfish (born 1899) & Lilienthal to Carlsen & Caruana (born 1992).

Viktor Korchnoi is the only man on earth beating every of the eight non-disputed post-war II world chess champions from Botvinnik to Kasparov in a regular classical tournament or match game.

Gallery of the Nine undisputed World Chess Champions, beaten in classical chess when they were Grandmaster at least once by Korchnoi:

Botvinnik, Smyslov, Tal, Petrosian, Spassky, Fischer, Karpov, Kasparov, and Carlsen
(who is considered as the 16th World Chess Champion since Steinitz in modern history)

In beating nine <undisputed> World Chess Champions in classical games, Korchnoi shares a unique record alongside Paul Keres, the other all-time-great uncrowned king of chess, and Alexander Beliavsky (with Viktor Korchnoi having the highest number of classical game wins against future, reigning or past World Chess Champions). As pointed out, Korchnoi is the only player to beat all eight consecutive World Champions from Botvinnik to Kasparov.

> Oldest great player against whom Korchnoi won (1953):
GRIGORY LEVENFISH, born 1889, GM 1950 (GM title inauguration)
> Youngest great player (ELO 2700+) against whom Korchnoi won (2011):
FABIANO CARUANA, born 1992, GM since 2007

Age difference between Korchnoi and Caruana is 61 and a half years, the same delta as it would be in boxing between Schmeling and Tyson 😉 

There are even later born players beaten by Korchnoi under regular tournament conditions in classical chess, but these players are at that time too young to be considered at very Super-GM level, among others: Hou Yifan, born 1994 (at Paks, Marx Gyoergy Memorial, her ELO was then 2523), or Lars Rindlisbacher, born 1996 (at the Legends vs. Geneva match in Geneva, that means a difference of 65 years to his swiss fellow who made then an IM norm).

Viktor Korchnoi, "the Rolling Stone of Chess" faced and beat players from at least five chess generations: from Levenfish & Lilienthal as well as Najdorf, Reshevsky, Keres and Botvinnik, to the coeval generation of Petrosian, Tal, Polugaevsky, Stein, Spassky, Larsen and Fischer to Karpov to Kasparov until the days of Carlsen & Caruana including among others Gelfand, Ivanchuk, Shirov, Svidler, Topalov, Leko, Judit Polgar, Vallejo Pons, Ponomariov, Bacrot, Grischuk, and Gashimov.

1. winning against the Pre-War warriors, already playing competitively before / during World War II, notable players faced and beaten by Viktor Korchnoi include among others mentioned Levenfish (born 1889), Lilienthal (born 1911); then Lisitsin, Sokolsky, Mikenas, Guimard, Barcza, Golombek, Rossolimo, Bondarevsky, Nezhmetdinov, Kluger, Stahlberg, Pirc, Prins, Paoli, Denker, Flohr (born 1908), Najdorf (born 1910), Reshevsky (born 1911), Keres (born 1916), and Botvinnik (born 1911)
2. winning versus all of the rising Soviet Post-War youngsters: Smyslov, Bronstein, Geller, Petrosian, Tal, Polugaevsky, Stein, Spassky, as well as versus Fischer, Larsen & Co. and all the best of the Rest of the World
3. winning versus the Karpovian generation and his seconds, with Karpov 20 years younger
4. winning versus the Kasparovian generation, characterized by its revolution of the openings, meanwhile Kasparov 32 years younger than Korchnoi — be aware, they met first (apart from a clock simultaneous) when Korchnoi was already older than fifty years
5. winning versus the Engine kids:

• beating with about 40 plus year-odds vs. elite players as Gelfand, Ivanchuk, Shirov, Svidler, Adams, Topalov, Leko, and Judit Polgar,
• beating with about 50 plus year-odds vs. elite players as Vallejo Pons (born 1982), Ponomariov (born 1983), Bacrot (born 1983), Grischuk (born 1983), Gashimov (born 1986, R.I.P. 2014), Carlsen (born 1990), and Caruana (born 1992).
   

Lifetime scores of Viktor Korchnoi:

Korchnoi (1931) has a plus lifetime - score against the world champions of "his generation": Petrosian (1929), Spassky (1937) and overwhelmingly versus Tal (1936).

Apart from the mentioned world champions Spassky, Petrosian and Tal, Viktor Korchnoi has also a (frequently dominating) plus lifetime - score in a <substantial> number of encounters, vs. most of the top-players of his prime decades, ie.:

Averbakh, Taimanov, Suetin, Gligoric, Szabo, Barcza, Ivkov, Filip, Reshevsky, Unzicker, Uhlmann, Matanovic, Benko, Donner, Geller, Stein, Polugaevsky, Gheorghiu, Najdorf, Panno, Mecking, Torre, Hort, Portisch (narrowly), Olafsson, Larsen, Ljubojevic, Hübner, Miles, Nunn, Speelman, Timman, Yusupov.

Viktor Korchnoi is even with Beliavsky, slightly behind Seirawan or Short (even in classical chess), and younger stars, with still a considerable overall plus score versus about forty years youngsters such as Akopian, Lautier or Piket.

Against the 20-years younger Karpov and the 32-years younger Kasparov he has a minus overall lifetime - score, also against older fellows Keres, Bronstein and Smyslov (not including Korchnoi's two wins in blitz at Herceg Novi 1970 balancing the score); and naturally Korchnoi is in lifetime scores behind 44-years younger Kramnik, and clearly behind Anand (no wins).

Korchnoi tied legendary Bobby Fischer (+2=2-2;+1-1 in blitz); he tied also Botvinnik (+1=2-1).

The following players, Viktor Korchnoi faced most frequently otb in classical chess:
Karpov 108x (+14-31=63), Timman 75x (+19-14=42), Petrosian 70x (+12-10=48), Spassky 69x (+20-16=33), Polugaevsky 60x (+21-8=31), Hübner 50x (+14-12=24), and Tal 44x (+13-4=27), according to Chessgames (including rapid and blitz games does not change the net balance, there were a few non-classical games reecorded in earlier days, ie. apparently not a single one between Korchnoi and Petrosian, versus Karpov +3-4=6, versus Timman +1-0=2, versus Spsssky +5-2=2, versus Polugaevsky +4-0=2, versus Tal +0-2=2).

Note: Chessgames Database may be incomplete. For instance from the legendary 1970 Herceg Novi blitz tournament, in total, Korchnoi achieved 14/22 points as clear third, behind Tal and the outstanding winner Fischer, above Petrosian, Bronstein, Hort, Smyslov, Reshevsky, Ivkov, amongst others, but listed are only six of his games (four losses, and two wins). From the next 1983 Herceg Novi blitz tournament, Korchnoi was clear second at 10.5/16 behind Kasparov, above Tal, Ljubojevic, Timman, Spassky, Larsen, amongst others, only two games (two losses) are listed in the cg. database.

*****

Trivia:
From the 22 competitors (including two substitutes), regarded at that time to be the strongest Chess Grandmasters, playing in the 'USSR vs. Rest Of The World: Match of the Century' at Belgrade in Spring 1970, Viktor Korchnoi beat during his career all ! 🙃other than himself, of course.

Against 17 of them (Larsen, Portisch, Hort, Gligoric, Reshevsky, Uhlmann, Matulovic, Najdorf, Ivkov, Olafsson, Spassky, Petrosian, Polugaevsky, Geller, Taimanov, Tal, and Stein), Korchnoi has a plus lifetime record, twice an equal score (against Fischer and Botvinnik) and only against two a negative lifetime record (Keres and Smyslov).

Most wins against world champions

Paul Keres, Viktor Korchnoi, and Alexander Beliavsky are the only chess players to have defeated 9 undisputed world chess champions in classical games.

• Keres defeated every world champion from Capablanca (3rd) to Fischer (11th): José Raúl Capablanca, Alexander Alekhine, Max Euwe, Mikhail Botvinnik, Vasily Smyslov, Mikhail Tal, Tigran Petrosian, Boris Spassky, Bobby Fischer.
• Korchnoi defeated every undisputed world champion from Botvinnik (6th) to Kasparov (13th), and also Carlsen: Botvinnik, Smyslov, Tal, Petrosian, Spassky, Fischer, Anatoly Karpov, Garry Kasparov, Magnus Carlsen.
• Beliavsky has defeated every undisputed world champion from Smyslov (7th) to Carlsen (16th), except for Fischer: Smyslov, Tal, Petrosian, Spassky, Karpov, Kasparov, Vladimir Kramnik, Viswanathan Anand, Carlsen.

Korchnoi and Beliavsky would both move to 10 if Fabiano Caruana were to win the World Chess Championship 2018.

If disputed world champions are included, then Garry Kasparov, Korchnoi, and Beliavsky share the record at 11. Kasparov has defeated 7 undisputed world champions (Smyslov, Tal, Petrosian, Spassky, Karpov, Kramnik, Anand) as well as 4 FIDE world champions (Alexander Khalifman, Ruslan Ponomariov, Rustam Kasimdzhanov and Veselin Topalov). Korchnoi defeated the 9 mentioned above, plus FIDE champions Ponomariov and Topalov. Beliavsky defeated the 9 mentioned above, plus FIDE-only champions Khalifman and Topalov. Note: classical games, no rapid, blitz or simuls.

If draws are included, Korchnoi has beaten or drawn 11 undisputed world chess champions in a row in classical games (draws against Anand and Kramnik) and all 4 disputed world champions (draws against Khalifman and Kasimdzhanov) – that means, every world champion since Alekhine's death in 1946!

Source: Wikipedia