If Steelix has to be included, D Rank (or below) is the best fit.
* He is immune to Electric, but vulnerable to Ground. Ground moves are often physical however, meaning that he can survive them unless they are STAB.
You are misunderstanding how Ground (and Fighting) moves affect Steelix's performance in OU. The fact that Steelix can survive super effective Physical attacks is not the key point here. The problem is that these attacks are hitting for SE, and therefore, are doing significant damage.
The most important thing to note is that
every physical offensive pokemon in OU relies on
Fighting, Ground, or Water attacks in order to complete coverage.
EVERY SINGLE ONE. There are no physical attackers in OU who don't hit Steelix for super effective damage-- AND it has no reliable recovery. Even though Steelix is far bulkier than Skarmory,
every physical Pokemon in OU can hit it harder than Skarmory just because of its weaknesses.
This means there are literally zero opponents Steelix can switch in on easily (enemy skarm/forry? They set up all over Steelix; physical jirachi? woo-hoo)-- a ridiculous problem for a defensive Pokemon.
A defensive Pokemon that isn't a full stop to at least a handful of threats is pretty much worthless.
Furthermore, SR and phazing alone just aren't enough support to justify a spot on a team-- not when competing against hippowdon, skarmory, and foretress.
Skarmory--
-Has no weaknesses to the majority of physical attacks, as physical fire/electric moves are very rare (and Victini/Darmanitan would destroy Steelix anyway; physical electric moves are basically non-existent).
-Roost gives it a lot more survivability (phazing is infinitely more useful on a Pokemon that has recovery)
-Spikes gives it something to do against the foes it walls (since it does wall some foes, unlike Steelix)
-STAB Flying is a MUCH better mono-STAB for a defensive Pokemon-- nothing resists it, it's a powerful offensive type, and BB has 120 base power. Big difference from Steelix's EQ, which almost half the meta is immune to.
Foretress--
-Has no weaknesses to common physical attacks
-Spikes is key; as Foretress almost always has a way to take advantage of the switches its defensive abilities force
-Rapid Spin also lets Foretress provide key support
-Even though it will eventually get worn down (like Steelix), its key support options let you get a lot of bang for your buck
Bronzong
-That immunity to Ground is sooooo clinche; Dragon + Ground, Ice + Ground, Rock + Ground, etc.
-Neutrality to Fighting and Water helps too
-Bronzong's defensive type allows it to STOP KEY THREATS that nothing else can.
-Compare this to Steelix being hit very hard by EVERY offensive Pokemon in the entire metagame
