Ok I'll give it a shot.
I'll rank riders based on big wins (GTs, Monuments, WC/Olympic RR).
Disclaimer 1: I'm reluctant to include WC/Olympic TT titles in this ranking because they're relatively young races, hence the greats of the past didn't have a chance to win them. Could be used as tie-breakers.
Disclaimer 2: races' prestige has changed over the years. Winning Fleche Wallonne or Paris-Tours was obviously rated higher 50 years ago than it is today, and that's why I decided to include only GTs/Monuments/WCs in this ranking. It's arbitrary, I know.
Disclaimer 3: I'm the biggest sucker for podiums you'll ever meet but I do believe podiums don't matter much if we're discussing the absolute best of the sport.
So let's start:
| GTs | Monuments | WC/O RR | Total | Notes |
Eddy Merckx | 11 | 19 | 3 | 33 | |
Fausto Coppi | 7 | 9 | 1 | 17 | |
Bernard Hinault | 10 | 5 | 1 | 16 | |
This is the elite of the sport, with Eddy being clearly a few steps above anyone else.
| GTs | Monuments | WC/O RR | Total | Notes |
Alfredo Binda | 5 | 5 | 3 | 13 | |
Gino Bartali | 5 | 7 | 0 | 12 | |
Costante Girardengo | 2 | 9 | 0 | 11 | |
Roger De Vlaeminck | 0 | 11 | 0 | 11 | |
Sean Kelly | 1 | 9 | 0 | 10 | |
Rik Van Looy | 0 | 8 | 2 | 10 | |
Felice Gimondi | 5 | 4 | 1 | 10 | |
Jacques Anquetil | 8 | 1 | 0 | 9 | |
This is the tier of riders I consider a lock for the top20. You can move Anquetil up a few spots if GTs are your thing, or Kelly/Van Looy in case you like Classics more. I personally believe Gimondi is probably the most underrated rider in the history of cycling, winning as much as he did while riding in the era of Merckx.
11 of the top20 spots are filled. 9 to go.
Now things get tricky as we move down the rankning. There are 5 riders with 8 big wins and 8 more riders with 7 big wins.
Here's the list. I put a few notes for tie-breaking purposes.
| GTs | Monuments | WC/O RR | Total | Notes |
Francesco Moser | 1 | 6 | 1 | 8 | 152 wins, 2 TA |
Rik Van Steenbergen | 0 | 5 | 3 | 8 | 2 FW, 25 GT stages |
Paolo Bettini | 0 | 5 | 3 | 8 | 1 TA |
Louison Bobet | 3 | 4 | 1 | 8 | 1 Dauphine, 1 PN |
Tom Boonen | 0 | 7 | 1 | 8 | 121 wins, 3 GW |
Fabian Cancellara | 0 | 7 | 0 | 7 | 6 WC/O TT, 1 TA, 1 Suisse |
Moreno Argentin | 0 | 6 | 1 | 7 | 3 FW, 15 Gt stages |
Henri Pélissier | 1 | 6 | 0 | 7 | 1 Paris-Tours |
Johan Museeuw | 0 | 6 | 1 | 7 | 1 Amstel, 1Paris-Tours |
Miguel Indurain | 7 | 0 | 0 | 7 | 2 WC/O TT |
| GTs | Monuments | WC/O RR | Total | Notes |
Alberto Contador | 7 | 0 | 0 | 7 | 4 PV, 2 PN, 1 TA |
Chris Froome | 7 | 0 | 0 | 7 | 3 Dauphine, 2 Romandie |
Vincenzo Nibali | 4 | 3 | 0 | 7 | 2 TA |
Moser and Bobet are in for me, winning all 3 types of big races.
Froome and Indurain are in too, because TdF.
Then there's a large group of Classic specialists I'd rank this way.
IN:
Boonen (best cobble rider ever, number of overall wins),
Cancellara (TT titles).
OUT: Van Steerbergen, Bettini, Argentin, Museeuw, Pélissier (not strictly a classic specialist, I know).
Still 3 spots to fill.
Contador and Nibali against a number of riders winning 5/6 big races. I'll name a few: Valverde, Rominger, Gilbert, Freire, Magni, Sagan, Lemond, Fignon, Bugno, Saronni...
Almost impossible to choose, so I'll take the easy way out and select the three riders with the biggest lobbies on this forum: Contador, Nibali, Valverde. Don't wanna argue forever.
Final list:
Eddy Merckx
Fausto Coppi
Bernard Hinault
Alfredo Binda
Gino Bartali
Costante Girardengo
Roger De Vlaeminck
Sean Kelly
Rik Van Looy
Felice Gimondi
Jacques Anquetil
Francesco Moser
Tom Boonen
Louison Bobet
Fabian Cancellara
Miguel Indurain
Alberto Contador
Chris Froome
Vincenzo Nibali
Alejandro Valverde