I must say these four are the best riders of the last 10 or more years by some margin. I would divide best cyclists of the past 10-12 years into three groups. In the first group are four mentioned above. In the second Nibali alone. And in the third we have Cavendish, Gilbert, Purito, Froome, Wiggins, Cunego, and certain Davide Rebellin.
As for the ranking of the best four concerns, that is a really hard task. The margins are very slim, and although I am a huge fan of Valverde I must give slight advantage to Alberto Contador, one of the greatest GT riders of all-time. His 7 Grand Tours (although I actually count 9) speaks for themselves. He stands right next to Merckx, Coppi, Hinault, Anquetil and Indurain. It's not just his wins, but the way he won them, legendary stuff!
From 2-4 are tiniest of margins I could imagine. Cancellara is my no.2 although I like both Boonen and Valverde more. He's very complete, his palmares is more than impressive! All that Monument wins, WC Time-Trial titles, yellow jerseys tips the balance in his favor.
Third is my favorite rider, one and only Don Alejandro "The Great"! What a talent, what class on the bike, what variety of different skills, not seen in the peloton for a long, long time. He would definitely be my first pick with only one or two really big wins. That's the only thing missing, and that could change in the next few years, given the way he's riding at the moment.
And at the end there's Tommeke, maybe the best cobbles rider of all-times. In top form he's an unbeatable force, you can't drop him and you can't beat him in the sprint either. He can win just as he likes, and he did just that, many times. Without that cocaine episode and unfortunate injuries he would stand even higher. Greatest talent after Valverde in the last 15-20 years in my opinion.