Steelix defensive moveset

Dragon Tail

Dragon Tail for being resisted by less Pokemon. Iron Tail is resisted by fire and water.

You also may want to consider Heavy Slam for the Charged Move. It hits neutral on fighting counters and super-effective on rock/ground counters.

Crunch will hit neutral on nearly any counter that a Steelix would face, and has 3 bars, but its damage is extremely terrible for the time to use it.

I know Heavy Slam is resisted by Fire counters, but it gains STAB, is a 2 bar move, and comes out pretty quickly. Earthquake seems like the optimal choice against Fire counters while also getting STAB, but the Steelix is unlikely to use it more than once in a fight and it is SUPER easy to dodge.

Dragon Tail - a neutral damage Dragon Tail will do more damage than a STAB + resisted Iron Tail, and Dragon Tail gives more energy per use.

Water and fire types resist steel moves while water and fire moves are also super effective against Steelix.