As far as numbers are concerned, then Loeb is the pick :

4 times Champion, 38 wins, best % of wins vs Starts, 17 different rallies won, that's 4 more than the second best : Sainz, and last, the perfect balance of 50% of his wins on tarmac and 50% on loose surfaces which betters by far all of the fantastic past champions.

But, like said different eras... and no comparison with F1, ain't no such different surfaces.

I think it'd be more interesting to determine an ultimate top ranking.

And who cares if whether or not one won his 2 titles with a 206WRC or with 2, 3, 4 different cars or took 95.8% of his wins in a Mitsubishi GPA or 100% in a Citroën ? That's BS excuses. :