Despite at first showing my opposition to Rotom-F being moved to S rank, I'm quite torn about it right now, but still more prone to wanting it be left in A+, as S would be quite a huge step for it, and therefore I would need to be completely sure about the matter before approving the nomination fully. Moving Rotom-F to the S rank depends on if its qualities heavily outweigh its negatives and how well it can abuse current metagame trends, so I'll focus on these two points. The post is long so I'll highlight my main points.
Starting off from its qualities, Rotom-F is mostly known for its BoltBlizzard STAB combo, which makes it hard to switch into, and allows it to break through common Electric-types checks and counters, naming some Stunfisk, Roselia, Golem and so on. This combo, along with high Special Attack and decent Speed, also makes it extremely threatening as a cleaner, when running a Choice Scarf set, against most offensive builds that don't pack one of the really few dedicated answers like Seaking and Chinchou. This set is also able to carry Volt Switch, which becomes extremely effective when running Rotom-F, given Blizzard scares out most Electric-type immunities, and therefore allows it to almost freely pivot in and out against bulky foes or Pokemon Rotom-F can't bring down straight off the bat. Another quality Rotom-F carries, makes it have even fewer checks and counters, and I'm talking about its versatility and unpredictability before having scouted its item. Rotom-F, other than the Choice Scarf set, can also run a SubWill-O-Wisp and a SubSplit set, both really threatening especially when running a slow-paced or unprepared team, making Rotom-F a true pain for balanced and defensive builds too, given it is able to reliably wear down bulkier foes, which in some cases can't even break through the Substitute, this because of Rotom-F's decent bulk even without investments. A Substitute set, as already said by other users, can also be deadly to offensive teams if they let Rotom-F set up one, and can lead them to having to sac at least one Pokemon before being able to dent Rotom-F health bar. Access to Levitate is also quite a feature in Rotom-F's kit, as it is able to provide offensive and balanced teams with one of the few Ground-type immunity, which I mean still gets steamrolled by Rock-type coverage, but can at least offensively threaten and revengekill Ground-types. Moving onto negatives, remember early when I talked about its combo? Well, the combo, despite being really good, also contains one of Rotom-F biggest flaws, which his Blizzard's accuracy: in fact, Blizzard has a shaky accuracy and missing with it isn't rare at all, this makes Rotom-F a less reliable revengekiller and cleaner, which are its two main jobs to put work in. Another pretty big issue Rotom-F has is its weakness to Stealth Rock. Despite not necessarily needing entry hazard removal, Rotom-F is still annoyed by the presence of Stealth Rock in the field, as they limit its potential in VoltTurn cores, and are able to decimate pretty quickly Rotom-F's health bar along with the usage of Substitute, therefore deterring the use of Substitute itself. Stealth Rock, along with Rotom-F decent but not exceptional / high Speed, also put Rotom-F in danger of faster Choice Scarf users such as Simipour and Dodrio, but also a plethora of other offensive Pokemon in case it isn't carrying a Choice Scarf. Overall I would say that Rotom-F is extremely slot efficient, and its positives definitely outweigh its negatives but not that heavily to blankly consider it S rank before going over other points.
This last point will be quite short because it's really late and I want to sleep. Rotom-F has risen as a top Pokemon pick in most teams almost purely because of metagame trends. First of all, the decline of Fire-types hugely helps Rotom-F: Fire-types such as Ninetales are decent checks to Rotom-F, even if they can get worn down quickly by Stealth Rock, but the same goes for Rotom-F, they resist Blizzard and most of them can outpace Rotom-F when it's not running Choice Scarf. Also Ninetales tanks a Thunderbolt even after Stealth Rock damage so it can check the Choice Scarf nicely too, not that sure regarding others such as Simisear though. Other metagame trends that help Rotom-F consolidate its dominance is that it deals well with bulky Encore users such as Audino used to deal with the popular Vigoroth, and bulky Pokemon in general that are used to check Exeggutor, such as Vullaby, bulky Zweilous, Tangela, and so on. Stunfisk, despite the presence of Exeggutor, is still kind of omnipresent on bulkier teams that want something more reliable defensively than Golem, especially against Monferno and Pawniard, so Rotom-F fares against that pretty well too. But, there's a but, with the suspect test coming to an end, we don't know what the result will be and if the metagame will have a different shape compared to the current one, so I would honestly wait a bit before taking a decision regarding Rotom-F's ranking, and that's also another reason for why I'm iffy about promoting it to S right off the bat.
