Steelix

Anyways, put it in a gym and it will be cool.

Easily countered by a 4-rated Pokémon, Azumaril, with Bubble/HP, which is also being a very decent prestiger, about 800 prestige can be easily gained in each battle only if you have the time.

Dragon Tail is superior to Iron Tail on defense.

I don't really think that moveset is markedly different on defense than the others. I think most folks will be running a Steelix out of the gym with a Vape. Although these moves don't get stab they also aren't resisted. So it's not that different really.

http://www.pogomoves.com/check.php?lang=en&pkmn=Steelix&basic=Dragon+Tail&charge=Crunch
On defense it is about 90% of the damage of its best moveset. The benefits are that Dragon Tail generates a little more energy and is not resisted by fire and water. Given that most people will be attacking with water or fire, it is probably pretty close to the steel movesets.

You can run some scenarios on pokebattler.com if you want a better impression before you invest.

Not really good, but dragontail does the same damage on defense because most likely it gets attacked by pokemon that resist steel. (water, fire)
Crunch is also not bad if it get's attacked by said pokemon,