Not actual skill, just (perception) you can change it as you see fit.
For me:
Tier 1: (Senna)
Tier 2: (Fangio and Clark)
Tier 3: (Schumacher and Prost) (Vettel and Alonso)
Tier 4: (Stewart, Lauda, Brabham, Ascari)
Tier 5: (Piquet, G.Hill and Fittipaldi) (Lewis and Kimi)
Tier 6: (Mansell, Haikkinen, Rindt, Andretti, Hawthorn, Farina and Surtees) (Jenson)
Tier 7: (J.Villeneuve, D.Hill, A.Jones, Scheckter, Hunt, P.Hill, K.Rosberg, D.Hulme)
For me:
Tier 1: (Senna)
Tier 2: (Fangio and Clark)
Tier 3: (Schumacher and Prost) (Vettel and Alonso)
Tier 4: (Stewart, Lauda, Brabham, Ascari)
Tier 5: (Piquet, G.Hill and Fittipaldi) (Lewis and Kimi)
Tier 6: (Mansell, Haikkinen, Rindt, Andretti, Hawthorn, Farina and Surtees) (Jenson)
Tier 7: (J.Villeneuve, D.Hill, A.Jones, Scheckter, Hunt, P.Hill, K.Rosberg, D.Hulme)