Originally Posted by Fat Ewil
for saying one, Regirock can't learn Rock Blast while Golem can, and with it he can sweeps or forces multiple Flying/Ice/Bug pokes most notably Ninjask. Golem has Sturdy to pratically guarantee Stealth Rock on the field or to take an huge damage and hit back in emergency cases (like an over boosted sweeper). Sucker Punch is not used to kill Psychic type principally, but is used as a priority move for weaked opponents. You hardly find yourself using Sucker Punch at the beginning of a match, SP is used in mid-late game if you decide to not sacrifice Golem once you set up your Stealth Rock.
The dual STAB of Golem is useful principally to hit neutral mons. I think it's obviuous that a SE Attack coming from a decent attack stat is going to OHKO a pokémon, especially Skuntank that usually runs no DEF EV.
Regirock is more saw as a utility mon that can actually do some damages to the opponent, that's why he usually runs things like T-Wave/Toxic+Stealth Rock+Rock Slide/Stone Edge+Filler, while Golem usually is used as a SR setter and bulky attacker since his usual set is SR+Rock Blast+EarthQ+Sucker Punch.
For the Eletric attacks, Golem is a check, like when you predict that the opponent's Rotom-F/S is going to locked itself in a Volt Switch/Thunderbolt and Golem can come in for free, that obviusly a base example, since i think that if your opponent see a Ground pokémon in your team, he/she'll going to not spam Electric attacks and that's another advantage to have Golem in your team (or a Ground pokemon in general).
So, both Golem and Regirock have their good points and the choose of one of them depends of what you need in your team.
OK, good luck sweeping with base 45 speed. Rock Blast is a boon for Golem, but Golem really shouldn't be your main check to substituting pokes anyway, as Rock Blast isn't guaranteed to hit 5 times every time. Relying on that WILL cost you many games, so it's whatever. Sturdy is irrelevant, as Regirock will get that soon anyway. Even then, it doesn't matter. Both Regirock and Golem are designed to take hits and tank back multiple times throughout the battle, which means the only time you'll have Sturdy up is when it doesn't matter one bit, like in lead matchups with Regirock, other Golem, Piloswine, whatever. Sucker Punch is unreliable priority too. It's fairly easy to predict around, and will fail against other priority users. Anything frail enough to be taken out by Golem's sucker punch can probably also be taken out by Regirock taking a hit and finishing it off. Not to say priority isn't good on it, but it's not enough to justify using it over Regirock.
Regirock can still pull off an attacking tank set extremely well, ESPECIALLY as it gets Drain Punch, something Golem lacks. Regirock is not outclassed at all in this role. However, Golem cannot pull a supportive set off, meaning Regirock outclasses it there. Regirock can even pull pseudo swords dance with swagger and pysch up (which with Musharna Trick Room support makes for some pretty lolzy matches). Golem has one set; bulky stealth rock tank. Everything else Golem does is done better by Regirock, and Regirock can do the one thing Golem can just as well. Regirock can also do things Golem cannot do, like support. There's one Golem set listed on his analysis, and there are three Regirock sets. That says a lot.
And as I said previously, Golem is not a fantastic check to electric types. Good players will never spam electric moves when Golem is around, and will instead destroy it with HP Grass, HP Ice, Giga Drain, or whatever. Whereas with Regirock, you can switch in with impunity, taking minimal damage from either a STAB electric move or a super effective HP (because Regirock actually can take special hits surprisingly well, with the same special bulk as Ludicolo).
Don't get me wrong, Golem is a good pokemon, but Regirock is just better. If you prefer Golem, that's fine, it can still function well. But Regirock will do most everything Golem does better than it, something which cannot be overlooked at high level play. You won't notice against your typical ladder player who has a team full of Kanto staples, but when facing a great opponent, I would pick Regirock every time.