X Dark Serge X
Well, somehow I´ve been lucky when it came to the guild steel trainers... there was but a few interested in the type... and now it suffered from a nerf in resistances... well, trading ghost and dark for resisting fairy... which I guess keep it somehow in the edge of being useful It's hardly a nerf. You trade 2 resistances for 1 resistance and something you can hit Super Effectively. Counting Offense and Defense it's 2 for 2, or as they say "Plus 0".
10 resistances, 1 immunity, 3 weaknesses.
Hits 3 Super Effectively, resisted by 4 types counting itself.
Steel IS the best type. Every team wants to have at least 1 Steel type, 2 if you can fit them. Weaknesses to Fire, Fighting, and Ground, all common attacking types, however, all 3 of these types are very easy to defend against. Flying resists Fighting and is Immune to Ground, Fire is resisted by Water which isn't weak to Ground or Fighting either.
Bronzong/Heatran/Empoleon/Skarmory/Ferrothorn/Forrestress are all seasoned defensive Pokemon.
Then you have offensive powerhouses like Metagross, Mega Mawile, Scizor, Lucario, Excadrill (Not Ubers thanks weather nerf), Aegislash, and the ever-useful Jirachi.
All of those Pokemon are OU (Except Bronzong and Empoleon). Icing on the cake is SR resist.
And before anyone points it out, yes Flying has plenty of OU Pokes too. Probably second after Steel. But we suffer SR weakness and there is another thing I'm not going to tell you, because it would give you too much of an advantage.
I commend you for using Steel because you like it instead of using it because it's the best. I use Flying for the same reason, even if's one of the more difficult types to master.
@Other types
People say Ghost is OP now. But it only hits Psychic/Ghost Super Effectively, immune to Normal, and can't hit Dark well. Defensively it has 2 immunities, 1 resistance, and 2 weaknesses. Pretty even if you ask me. If you have too many Ghost weaknesses just run a Dark or Normal Pokemon.
Dark got nerfed anyway. It can hit Steel neutrally but can't hit Fairies well and gained a weakness defensively to Fairies.
Fighting/Dragon got nerfed by definition.
Poison got a boost, which it needed.