I voted for:
1: Steelix
2: Mawile
3: Jirachi
4: Ferrothorn
5: Aegislash
6: Klefki
My favorite Steel type pokemon is definitely Steelix, has been since Generation 2. It has the bulkiest defense stat, it has in my opinion one of the single best type combinations in the game, it has a fantastic move pool with great abilities to boot, and they even gave it what I think is an amazing looking Mega. Steelix is fantastic and I don't see that changing anytime soon.
Honerable Mention: Mawile
I like Mawile a lot as well and I have liked it for much longer than it's incredibly powerful Mega and typing was released. I used to be obsesed with Mawile for a long time due to the fact that it was (previous to generation 6) a Solid Steel type pokemon. I think that the Steel type with no other type combination is an absolutely fantastic type, and Mawile was until Generation 5 the only one. I was a little disappointed to see it lose that solid type in order to gain Fairy, but I was happy to see a pokemon that I always enjoyed in the past to get the power and respect that I felt it always deserved.
Ironically, I'm not a big fan of the Klingklang family. I've just always found it a strange pokemon and don't like using it.