Gilbert's certainly had the best year so far of all the riders on the mens tour. That's not up for debate, although if Bertie wins the Tour, that eclipses Gilbert's achievements.
You have to look a little beyond that though to say who the best rider in the world is - the last 2 or 3 years seems a fairer way to do it. Looking at the palmares of the big 4 over the last 2 and a half years...
Gilbert - 3 Monuments, 4 other Classics, 2 Vuelta stages, 1 Tour stage
Cancellara - 2 Monuments, 2 World TTs, 3 Tour stages, 2 Vuelta stages, 1 Tour de Suisse win
Cavendish - 1 Monument, 11 Tour stages, 5 Giro stages, 3 Vuelta stages and a points jersey
Contador - 2 Tour wins, 1 Giro win, 1 win each of Catalunya, Pais Vasco and P-N.
I don't think Gilbert compares favourably with any of the other 3 based on this. Comparing Gilbert to the other great Classics riders in history, in terms of Palmares, and then comparing the above 3 with the other great TTers, sprinters and GC men, respectively, shows that Gilbert has some way to go, but the other 3 are probably right up there already.
In summary, the best rider so far this year, but probably only the 4th best rider in the world, looking objectively.