Gengar from
A- to A
Before I go into the sets Gengar currently frequents that warrant the rise to A, I want to point something out about the tier itself that plays into the hands of Gengar itself.
Nobody uses any Ghost resists. Look at the Viability Rankings -- besides Greninja (and Ash-Greninja) which is (are) incredibly frail, there are no Ghost resists at all until B+ rank. Needless to say, the current environment of the SM OU metagame is very much favorable to Ghost types and Gengar is the best of the lot.
Gengar uses three main sets -- Scarf, Specs (can just be LO Offensive), and Ghostium Z. Scarf Gengar is pretty solid due to the spammable STAB it has and the good speed tier, but it's nothing exceptional. Moreover, I will not be elaborating on this set, but it surely adds an extra element to the overall viability of Gengar.
Specs Gengar was initially used by ABR during
his week 2 SPL game against Ciele and, since then, it has gained a fair amount of popularity. While it is far from being the fastest or strongest Pokemon in the tier, it is incredibly hard to switch into for almost all teams and it gets in more than one would expect a Pokemon as frail as Gengar to safely get in. Shadow Ball, Sludge Wave, Trick, Focus Blast, Hidden Power (Ice), and even the odd Thunderbolt are viable on this set and it tends to break apart balance teams while always doing a ton against offense when it gets into the game. This set can also run a Life Orb over Specs and then it has a whole boatload of utility moves in the last moveslot or two, but it sacrifices a bit of power and with Shadow Ball being so spammable, it's fairly easy to see why Specs is trendy right now. Overall, this variant of Gengar is so problematic to practically every team and there are few, if any, things in any rank that can do this, let alone A- or A (obviously Pokemon like Mega-Metagross and Pheromosa are consistently threatening, but they're very different and ranked highly for many good reasons -- Gengar simply deserves A due to the sheer effectiveness it's typing and what it has at it's disposal grants it).
Z-Shadow Ball Gengar is a bit of a different dynamic than Specs in a sense that it can bluff Scarf, switch moves, and make more use of Gengar's movepool. Personally, I run Taunt / Shadow Ball / Sludge Wave / Wisp or Focus Blast, but there are a couple alternatives in the last slot if you wish to deal with specific things better. FV uses this set during Week 3 of SPL against ABR, but it didn't get a ton of use until the game was already over; however, I used it this past week against Hector and it worked wonders for me early game. This set isn't as consistent or straight-up potent as Specs, which is why I will emphasize a bit less upon it, but I do feel the sheer utility of such a powerful and sometimes unexpected Z-Shadow Ball is dangerous in a tier lacking resists to it and Taunt is also quite helpful. Additionally, not taking LO recoil is great against bulkier teams, especially Stall, and the aforementioned bluffing aspect can often come into play when using this set. Moreover, this set is another thing that distinguishes Gengar, making it incredibly hard to counter and quite unpredictable.
Ultimately, Gengar has been very successful at higher levels of play - it has roughly 9% usage in SPL, but it has won 77% of the games it has been used in, which stands out like a sore thumb for a Pokemon sitting inside the top 20 overall in usage. It is clear that there is something great about Gengar right now that contributes to this success and it mainly is the aforementioned outlook of the tier in regards to the lack of Ghost type defensive counter play. Pokemon like Weavile and even Tyranitar who often spelled doom for Gengar with Pursuit now hardly see the light of day and the general absence of this contributes to the landscape of the tier significantly.
Seeing as various top of the line Pokemon reside in the A tier while some sporadically used and effective Pokemon like Manaphy (former, mainly) and Mega Venusaur (former, mainly, again, but also a bit of the latter given the way the metagame is shaping and other Megas are becoming ideal on almost all teams) reisde in A-, it is clear that Gengar belongs in A tier and should rise in the next update.
