This is very difficult.
There are several names I feel you cannot seriously exclude:
- Alberto Contador
 
- Chris Froome
 
- Paolo Bettini
 
- Tadej Pogacar
 
- Lance Armstrong
 
- Tom Boonen
 
- Fabian Cancellara
 
Then you have 3 spots left to distribute between:
- Alejandro Valverde
 
- Peter Sagan
 
Vincenzo Nibali 
Philippe Gilbert 
- Mark Cavendish
 
Mathieu Van der Poel 
Primoz Roglic 
Roberto Heras 
I don't think Gilbert did enough besides being very versatile and the trajectory his career followed put even that in doubt. He won Lombardia when it was easier, never challenged in LBL again after 2011 and only saved his career after going to QST and becoming a pretty different rider. I'll omit him.
Heras won the Vuelta four times but did  not do much besides. I'll omit him too.
Van der Poel has had a stellar last few years but he's still pretty far behind Boonen, Cancellara and Bettini. Pogacar has been more versatile on the road (I know MVDP does cross and MTB and I don't care).
At this time I omit him too but he might very well make it in the coming years.
Still five names left: Valverde, Nibali, Roglic, Sagan and Cavendish.
Cavendish is in. He basically peaked as high as a pure sprinter ever could.
Roglic imo is out at this time. I don't foresee Valverde and Nibali still considered in fifteen years but for now their careers still outweigh that of Roglic in my opinion.
Question becomes Sagan v. Valverde v. Nibali for the last two spots.
In the end I am going to lean towards Valverde and Sagan here. Mostly for Valverde's consistency and Sagan's streak of WCRR's and green jerseys. I will say though that Sagan never really managed to outright beat the previous generation before they retired and only partly capitalized on the weaker interim generation before the current freaks took over.
Nibali had a great career but I still feel he somewhat overperformed. Apart from perhaps 2014 he was mostly playing second fiddle to Froome and Contador in GT's. If he had won Rio 2016 things might have been different.
Some honorable mentions
- Oscar Freire
 
- Remco Evenepoel
 
- Jonas Vingegaard
 
- Alessandro Petacchi (only exceeded by Cavendish in  GT stages won)
 
Final ranking
- Alberto Contador
 
- Chris Froome
 
- Lance Armstrong
 
- Fabian Cancellara
 
- Tom Boonen
 
- Paolo Bettini
 
- Tadej Pogacar
 
- Alejandro Valverde
 
- Peter Sagan
 
- Mark Cavendish:He should be on the list (for now) but not higher than 10. Too specialised.
 
I don't think any of the current classic stars did quite enough to elevate them beyond the three best GT riders so far (Contador, Armstrong and Froome). Of those three Armstrong was the most dominant but he didn't care for any other races. Froome was perhaps also more dominant than Contador but the latter was up there more often in one-week races and didn't require as strong of a team.
The three classic stars are a bit of a toss-up for me.
The rankings right now is just a waiting room for Pogacar to become nr. 1 anyway.
I could see people replace any of the last three with Roglic, Nibali, Gilbert or Van