Definitely Steel. On average, types have 3.58 resistances or immunities. Steel has 11. On average, types have 2.71 weaknesses. Steel fits the average with 3.
So what if steel moves are bad offensively? The true strength of a steel-type shines when paired with other types--Heatran can still use fire STAB, Skarmory still has flying STAB, and Ferrothorn can use grass STAB.
In XY, it's been strongly hinted and rumored that Fairy will be weak against Steel. Yikes.
I think claiming that Dragon is the best type is grossly exaggerated because it's really their high BSTs that made the type overpowered. Consider: Altaria has the same BST as both Alakazam and Ferrothorn. Altaria also has access to the most useful dragon type moves: Outrage, Draco Meteor, and Dragon Dance. And yet, Altaria is NU whereas the latter are OU. Meanwhile, the majority of OU's dragons have 600 BST.
Sure, dragon is useful considering it's only resisted by one type in the game (*cough* steel), but no one ever carries dragon coverage moves. I mean, when was the last time you saw Hidden Power Dragon being used for its good neutral-coverage? Defensively, you'll find that dragons fit the average. It's the stats that make dragons useful, not the typing per se.