The best moveset for Steelix in the current competitive meta typically features the moves Heavy Slam, Protect, Wide Guard, High Horsepower.
In the Pokémon Champions meta, Steelix can be countered by exploiting its weaknesses (fighting 2x, fire 2x, ground 2x, water 2x) and using effective check/counter teams.
The best item for Steelix is typically Steelixite, based on high-level tournament and ladder stats.