Wiggins has said in the last day or so that if he hadn't come through the GB system then he probably would have had to have doped to get to the top. British Cycling have had a very rigorous non-doping policy for years and the top road riders have all come through the track setup run by Dave Brailsford with a few exceptions.
that's why you're seeing them at the top - it's down to the training methods that DB and his team have adopted, rather than doping them up.
and as cougs says, these guys aren't knocking out the power that the dopers did and are competing against other non-dopers (with a few exceptions who are hangovers of the old days like Contador and Vinoukorov) so it's a more level field