Verde let me try to help you here. The way viability rankings is that everything is relative. Pokemon that are better than others are going to be higher rank. Pokemon that heavily affect the metagame are going to be ranked the highest. So, what is Stunfisk's niche? It's a Stealth Rock user that has decent bulk. That's all it has going for it. Stunfisk has unusable offensive stats, which may have been acceptable for a wall, but Stunfisk doesn't even have recovery, so it fails to fill any role other than a tank. Do Electric teams need a tank? Sadly, the answer is simply no. The Electric typing is peculiar in that it only has one weakness: Ground. Every Electric team is going to field the powerful defensive core of Rotom-W and Zapdos, both of which are immune to Electric's only weakness. As a result, there is no room for a tank that has no offensive presence and no recovery. A tank cannot have no offensive presence because otherwise it switches into attacks, takes damage, and does absolutely nothing to threaten the enemy. However, as mentioned above, Stunfisk has access to Stealth Rock, which is a hugely important utility move. The issue, though, is that Stunfisk literally cannot do anything else. The fact that Stunfisk has no recovery and no damage means it is a completely useless Pokemon outside of using Stealth Rock. I don't even need to compare Stunfisk to the far superior Alolan Golem.
I mean, just look at the ORAS Viability Rankings, when Electric didn't even have Alolan Golem as an option. Stunfisk was still D rank. If Stunfisk was D rank with no competition in ORAS, how could it possibly be any better in SM now that it does have competition in a comparative ranking? (For the record, Stunfisk's HP stat is too high to effectively use Pain Split, unlike Rotom-W)
Anyway, as I stated, viability rankings is all relative. Let's compare Stunfisk to the C rank Pokemon: Eelektross, Electivire, Galvantula, and Luxray.
Eelektross is actually a tank, unlike Stunfisk. Its access to Levitate makes it have 0 weaknesses and it can use Assault Vest to be bulky. It carries great utility in Knock Off, which helps significantly against the Normal-type defensive core that Electric often has trouble with. However, Eelektross's base stats aren't particularly good and it lacks damage (just like Stunfisk does), so it's only C rank.
Electivire is a (rather poor) physical attacker and is more or less outclassed by Alolan Golem. However, Electivire has the almost unique ability to use Wild Charge and Cross Chop to 2HKO all parts of the Normal-type defensive core. Because it is useless outside of this one niche, it's understandably C rank.
Galvantula creates an entire archetype of its own: Sticky Web. Of course, because Electric teams have access to Surge Surfer, there's no need for that sort of Speed control based on a rather terrible Pokemon when Alolan Raichu can do much, much better. However, that niche is enough to make it C rank.
Luxray is similar to Electivire in that it takes out the Normal-type defensive core. The difference between them is whether you value Electivire's Speed or the utility of Luxray's abilities.
These are all Pokemon that are generally outclassed by others. Eelektross is simply worse than Zapdos, Electivire and Luxray are worse than Alolan Golem, and Galvantula is worse than Alolan Raichu. However, they have a specific niche they are able to use. Stunfisk literally doesn't have a niche. Alolan Golem does Stunfisk's only job but much, much better. Because Stunfisk can't even make a specific niche for itself, it's D rank.
Wait, there is one thing Stunfisk can do: allow Alolan Golem to not use Stealth Rock! Yeah, that's when you know. Stunfisk is D rank at best because it opens a moveslot on Alolan Golem, which honestly is almost never ever worth the teamslot. I'd consider unranking Stunfisk completely because it just isn't viable in the current metagame. I honestly don't even need to compare Stunfisk to Alolan Golem; even without the presence of competition, it lacks the ability to stand on its own against more useful teamslots.
I appreciate your interest in contributing to the rankings, but I must insist that you become more familiar with Electric teams and Monotype as a whole before making such strong claims as "we don't know". Stunfisk has been discussed in the past and it's not going to be moving up. Everything Stunfisk can do, another Pokemon can do so much better. Please view the rules of this thread and keep them in mind. Do not post about Pokemon that you do not have enough experience with.
Edit: a word
