This completely differs from my opinion last gen, but I've "seen the light", so to speak. Victini is incredibly strong in the tier, and if played correctly, can be one of the most threatening pokes for a number of reasons. I'm going to spend the first part of this post doing a bit of a c/c between Darm and Victini (imo Victini is more than just a little bit better), and the second part discussing why exactly I think Victini is such a threat.
Alright, so, the thing about Darmanitan is that he's extremely one-dimensional. Although he technically has a "viable" Scarf, LO, and Band set, the ultimate purpose of all of these sets are the same thing. Hit things hard, hit things fast. There's a reason why his April Fools' Day sprite said "Click Flare Blitz" - that is what you're going to be doing with him 95% of the time. Sure, he gets decent coverage and shares Vic's access to u-turn, but Flare Blitz spam is what makes him good.
Similarly, Victini has V-Create, a tremendously powerful nuke that it too will be using quite a lot. The negative effects of using V-Create are arguably a little less bad than those of Flare Blitz - while dropping defenses and speed may seem like a death sentence for an offensive poke, there are very few offensive mons that can freely switch into a v-create, especially without set knowledge. As such, most the mons that are willing to switch into Victini are bulky and rather slow, and will either get 2HKOd by Victini's moves, or force it to switch - in either case, the stat drop is practically meaningless. By contrast, Flare Blitz puts your HP bar on an even smaller timer than it already is, which, combined with hazard damage and shitty defences, can quickly wear Darmanitan down to the point where it can be picked off by priority.
More importantly, though, Victini is anything but one-dimensional. Although there are certainly other viable sets, the ones that I've seen and used to the greatest effect are as follows: Scarftini, Bandtini, Physical LO Tini w/ GK, Fully Special LO tini, Expert Belt Physical tini w/ GK, Expert Belt mixed tini w/ GK, Final Gambit tini, Charcoal Mixed Trick Room Victini, LO Bulky Physical TR Victini, and Mixed Work Up Victini. These sets aren't just theory either; they're all used, and used quite commonly, especially in higher level play. The element of unpredictability that Victini brings to the table is actually far greater than most pokes with variable sets - even, perhaps, Mew. The way that most Mew sets work are that if you see one move, you know them all, or at the very least, you have a reasonable idea of what can safely switch into it. By contrast, Victini can feasibly run any combination of moves, both special and physical, and with insane coverage. As such, there are very few - or arguably no - safe switchins to Victini without knowing its set, and those that are (read: Snorlax) have dropped precipitously in popularity.
Sure, Victini has its drawbacks. However, these are actually not quite as potent as they might seem. Let's review two of the main arguments as to Victini's weaknesses.
1. Hazards: There's no denying it, Victini does hate hazards. It's a grounded fire type, which means it has the unfortunate property of taking up to 50% from entry hazards, which, combined with Victini's nature as an offensive mon who likes to use U-turn and LO, can rack up very quickly. However, hazard control is by no means restrictive as it was last gen. No longer are people forced to devote an entire teamslot to disgusting crap like Foresight Hitmontop just to get rid of hazards so their Tini can sweep. The amount of viable defoggers and rapid spinners who also carry tremendous damage, utility, or tankiness as well has multiplied enormously - and plenty of them are good partners for Victini, as well. There's a reason we've seen a rise in people using double or even triple fire comps recently - the fire types in the UU tier are some of the most devastating pokes around, and, to compensate, they're weak to hazards. However, now all that's necessary is to run a Forry or a Mew and it's perfectly viable to run a team that can overwhelm the fire counters on the other person's team and easily sweep.
2. Weak to dark "priority" (Sucker Punch/Pursuit): I'd like to address something about Sucker Punch/Pursuit mons that's been bugging me for a while. The implied consensus in most discussions about these mons (especially Honchkrow) is that the guy running Pursuit and Sucker Punch is always going to magically make the correct prediction. Here's actually what happens.
Honchkrow has a choice to make. It can either Pursuit, predicting the ScarfTini to switch out after it killed another member of its team, or it can Sucker Punch, predicting Victini to stay in. Assuming that both of these scenarios are equally likely, we can then see that there are four possible scenarios.
- Victini goes for the V-Create, and Honchkrow Sucker Punches. Honchkrow kills, obviously, and Honch gets a moxie boost, whereas Victini just goes RIP. Win/Lose
- Victini goes for the V-Create, and Honchkrow mispredicts and Pursuits. Victini goes first, and Honchkrow dies miserably in a flaming V. Lose/Win
- Victini tries to switch out, and Honchkrow Pursuits. Honchkrow kills the Victini, gets the moxie boost, and life goes on. Win/Lose
- Victini tries to switch out, and Honchkrow Sucker Punches (fkn idiot), hitting nothing at all. Victini's team gets free momentum, and Honchkrow is forced to switch out (probably). Lose/Win
Although the relative effects of each of these scenarios is debatable, on a simplistic level the whole "Pursuit-trapping" thing will only work 50% of the time. Given that two of the scenarios involve Victini dying, and the other two involve Honch dying or losing momentum, it's sort of silly to contend that Pursuit-trapping is always a winning strategy for the Honchkrow. As such, this really can't be considered as a "counter" to Victini, given that there's a 50% chance it'll fail miserably and end up killing the Honchkrow instead of the Victini.
At the end of the day, Victini is just incredibly good. There's very little that can deal with it directly as a counter, and the playstyle/offensive counters of hazards and pursuit trapping are shaky at best, especially with the changes in generation six. Because of these things, Victini definitely deserves to be S Rank. Just because a pokemon technically has weaknesses doesn't make its otherworldly strengths any less otherworldly.