The question is whether you just look at it mathematically in terms of UCI points, or also at the way these races were won. If somebody finishes solo in Milan-Sanremo, Gent-Wevelgem and the World Championships, and wins a bunch of other races, it would be fabulous. If a sprinter like Cipollini wins these races you can say, Nah, it's just a sprinter, he's only good in the last 200 meters and his team does all the work.
My vote also goes to Gilbert in 2011. Pantani didn't even lead the UCI rankings in 1998; Bartoli did. However that Giro-Tour double remains legendary in the memory.