No matter how overpowered any one type is, mono-type teams are typically fairly easy to counter.
This. Not since Psychic in gen 1 has any type really been overpowered. Dragon has generally been good, but it did have that nasty Ice weakness, not to mention Dragon itself, and now it has Fairy as well as a hard counter both offensive and defensively (although Fairy does lack any really good pokemon except Xerneas whose going straight to Ubers and maybe beyond).
If you're interested in sticking to one type, a weather team is generally the way to go, and even then you still want a reasonable amount of type diversity. E.g. Sandstorm is easier to build because three types are immune, but you'd still want something to counter major threats even if they aren't Steel/Rock/Ground type.
Edit: This isn't to say no types are generally better than others. Dragon, Water, Steel have all been mentioned and all are strong types, due to various factors (Dragon I mentioned above, Water has only one weakness, good pokemon etc., Steel has a huge range of resistanced, but a few weaknesses to common offensive types hold it back from being OP) --- "When playing a game, the goal is to win, but it is the goal that is important, not the winning." -Reiner Knizia