For some, this won't even be a debate. After all, while there are many ways to slice through the mountains of data that surround professional cycling, pretty much any list of significance you can put together is going to be topped by Eddy Merckx.
But we're open to the possibility that there might be a little bit more to it than that. Here, then, is a brief look at some of the greatest in history, and a few suggestions as to why each one might be the best of all.
11 wins (Tour 1969, 1970, 1971, 1972, 1974; Giro 1968, 1970, 1972, 1973, 1974; Vuelta 1973)
For the encyclopaedists at the renowned procyclingstats.com website, this probably wouldn't be a debate. Their ranking system has Eddy Merckx streets ahead of his nearest rival, Bernard Hinault, and frankly, it's pretty hard to argue.
The Belgian has the most career victories in history (525 as a professional and amateur); he won 28 Classics, three World Championships, and held the Hour Record between 1972 and 1984. What is more, he is the only man to have won all three main jerseys at the same Tour, in 1969.
Most importantly though, he has perhaps the most enviable Grand Tour record, winning all the of three of them, the Giro d'Italia (five times), the Tour de France (five times) and the Vuelta a España (once).
And all, reportedly, despite a heart condition (opens in new tab) - non-obstructive hypertrophic cardiomyopathy - which these days would probably prevent him from racing.
10 wins (Tour 1978, 1979, 1981, 1982, 1985; Giro 1980, 1982, 1985; Vuelta 1978, 1983)
Step forward 'The Badger', who finished either first or second in every Tour he completed - perhaps unlike Merckx, he didn't need to win every race he entered, but when he wanted to win, he tended to find a way to do so. And never was that better illustrated than in his final Grand Tour victory, the Tour of 1985.
Greg LeMond may feel that he could have made up the few minutes that separated him from his team-mate had the American been allowed to attack Stephen Roche on stage 17. The simple fact, though, that the Frenchman was able to battle through the race's last week after suffering a broken nose in a crash three days before stands as testament to his grit.
Hinault retired just a year later, at the relatively early age of 32. Past his best? Hardly - he had just finished his final Tour second only to LeMond.
>>> The Hour Record: An interactive timeline
Eight wins (Tour 1957, 1961, 1962, 1963, 1964; Giro 1960, 1964; Vuelta 1963)
Another to have won his first Grand Tour, with victory in the 1957 Tour de France, and another not to finish off the podium when making it through the race. Unbeaten in six top-level rides between the 1961 and 1964 seasons, he had a style and grace on the bike that few have matched to such success.
'Monsieur Chrono', the king of the time trial, was also a holder of the Hour Record, taking over from Fausto Coppi on his third attempt after posting 46.159km in June 1956. And throughout a career that featured a significant level of doping controversy, he maintained a taste for the high life: "To prepare for a race," he once said, "there is nothing better than a good pheasant, some champagne and a woman."
>>>What is it like to ride in a team time trial? Cofidis shows you (video) (opens in new tab)
Seven wins (Tour 1991, 1992, 1993, 1994, 1995; Giro 1992, 1993)
The only man to win five Tours back-to-back, 'Big Mig' took a little while to come to the boil compared to others, with his first Grand Tour victory coming at the 1991 Tour de France, six years after he had first entered the Vuelta.
For the next five years, though, he was almost unassailable, with his only defeat in eight outings coming in the 1994 Giro, where he came third. His was an era which rewarded the racing against the clock in which he excelled, with many Tours featuring more than double the amount of time trialling than more recent editions.
The youngest man to lead the Vuelta (at the age of 20), and yet another man to post a record distance for the hour - his 53.040km in 1994 beat the mark of Graeme Obree - his light shone relatively briefly, but fiercely bright.
Seven wins (Tour 1949, 1952; Giro 1940, 1947, 1949, 1952, 1953)
Five times a Giro winner, Fausto Coppi won his first title at the age of 20, before his career was interrupted by the Second World War. He was also deprived of a meaningful shot at the Vuelta, which until 1955 was not even an annual race, and had seen its early years dogged by Spanish parochialism.
In 1942, the Italian set his own Hour Record at 45.798km, a figure which would stand for 14 years until the advent of Anquetil. He is commemorated in the Cima Coppi, a title given to the highest peak in each year's running of the Giro.
Seven wins (Tour 2007, 2009; Giro 2008, 2015; Vuelta 2008, 2012, 2014)
Alberto Contador's victory at the 2015 Giro d'Italia (opens in new tab) took him to seven Grand Tour wins, although the Spaniard has celebrated nine times atop a Grand Tour podium, with the 2010 Tour and 2011 Giro titles stripped after a doping ban. He was the youngest man to win all three Grand Tours (completing the set in 2008 at the age of 25).
He won the Giro and the Vuelta in the same year (in 2008), but between 2015 and his retirement in 2017 he was unable to add another title - which would have taken him ahead of Indurain and Coppi in terms of total Grand Tour wins.
A divisive figure, Contador nevertheless used his attacking flair to become the greatest Grand Tour rider of his generation and certainly worthy of being mentioned among the all-time Grand Tour greats.
Five wins (Tour 1965, Giro 1967, 1969, 1976, Vuelta 1968)
The great Italian perhaps would have been higher up this list had it not been for the dominance of Merckx through the late 60s and early 70s.
Gimondi finished second to the Belgian three times in Grand Tours and finished behind him on numerous other occasions, taking runner-up spot to him in the Tour in 1972 and in the Giro in 1970 and '73.
Still, Gimondi was able to record five Grand Tour wins, including taking three pink jerseys in his home race. He won his first three-week race at the Tour at the tender age of 22, but was unable to take another yellow jersey, finishing on the podium only once more.
He holds the record for most podium finishes at the Giro d'Italia though with nine between his third place finish in 1965 and his final title in 1976.
Five wins (Tour 1938, 1948; Giro 1936, 1937, 1946)
Gino Bartali's rivalry with fellow Italian Fausto Coppi may have divided the country, but it drove the sport. Another man whose palmarès would surely have been even more impressive but for the war and the initially inward-looking nature of the Vuelta, he was a Giro winner in 1936 at the age of 21, defending his title a year later and then picking up the Tour de France in 1938.
His staying power saw him through 22 Grand Tours, 20 of which he completed, and his only two non-top ten finishes came in the final years of his career as his 40s approached. His crowning achievement, though, may have been the three consecutive mountain stage wins in the 1948 Tour, a record as yet unmatched.
Five wins (Tour 2013, 2015, 2016, 2017, Vuelta 2017)
Since taking a first Grand Tour podium in Vuelta in 2011, Chris Froome has become the dominant Grand Tour rider of this decade.
With an exceptionally strong team behind him, Froome has taken four titles in the Tour de France and added a Vuelta title in 2017. Not only that, the Briton became the first man since Contador in 2008 to win two Grand Tours in the same year with the Tour/Vuelta double.
To add to that, Froome has finished on the podium on four other occasions at Grand Tours; second at the Tour de France in 2012 and second three times at the Vuelta in '11, '14, '16.
While Froome may be some way off topping this list having never targeted the Giro, he is the only rider aside from Contador to have won the Vuelta in perhaps its more difficult incarnation at the back-end of the year rather than in spring time.
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