In my opinion, all time greats if they retired today would be Contador, Boonen, Cancellara and Friere.
I consider Contador because he is 1 of 3 cyclist who have won all the Grand Tours, and eventhough it didnt include the tour, he did a rare thing with winning 2 Grand Tours in a year (10 or 11 cyclist have done that). Also, I take into account all the weeklon stage races he wins, including Paris Nice, etc. I have more respect for riders that go to these races to compete, not to "train". These races deserves the respect of being treated as an accomplishment, without them you would have nothing to warm up for the major objectives.
THe closest one to me is one we tend to forget is Petacchi. Even with losing 6 wins from 06 Giro, he has 22 Giro stage wins, 20 Vuleta stage wins, 6 Tour wins, winning the point jersey in each race, and a Milan Sam Remo. the only reason he is not in the list is just could use 1 more single day classic win. A WC, another San Remo, heck even a Gent Wevelgem, HEt Volk or another Paris-Tour would be nice.
OThers that are close
GIlbert- I think everyone thinks of last year, but even before that he already had 2 Lombardia's, Amstel Gold, stage win in Giro and Vuelta (2), 2 Het Volks, 2 Paris Tours, several podium in the monuments. I think another major win or 2 and i would push him into the all time great.
Hushovd- 1 WC, 10 TDF Stage wins, 3 Vuelta stages, 1 Giro, 2 grand tour points jersey, Gent-Wevelegem, 1 Het Niewsublad (I still want to call it Het Volk) Paris-Roubaix podiums. Similiar to Gilbert, 1 more major win or a few more grand Tour stage wins I would push him over.
Rebellin- Yes he is active, eventhough with a smaller team, and of course he has the major wins, and even a 6th in the Giro back in 96. But I think he would need at least 2 major classic wins, which i doubt he will even participate in 2 classics.
Valverde- Because he was recently suspended, we forget what he accomplished. 1 Fleche Wallone, 2 LBL, 2 Dauphines, 1 Vuelta (1 2nd and 1 3rd) numerous podiums in the Ardennes and the WC. He could use another Grand Tour or a couple more hilly classic win or a WC
Evans- This is where I really like the fact he is always competitive no matter what race, like Tirreno-Adriactico, ROmandie, Dauphine, etc. I think another TDF podium, or a successful classic or maybe even winning like Paris Nice and Dauphine Libre could put him there. MAybe even a podium at another grand tour also, I need ot think abotu this a little more.
Somebody mention Jens Voigt, and that is an interesting suggestion, because of his domestique work and he had some nice wins, a couple of TDF stages, 5 Criterium INternationals, 2 Tour Of Germany when it was on the calendar. Instead of all time great, he would would go down as all time teammate. Which in this category the other consideration Hincapie also just because of him riding 9 TDF winners, and always being competitive in the Cobbles.
The next wave of cyclist on this list in no particular order without doing too much moredeep though would be Menchov, Basso, Cunego,