Well I quickly voted no, I run offense and thought wtf kangashkan sweeps like half the time and we have fucking genesect running around, who got two new op move in shift gear and e speed. Then I read the thread (yeah I was a dumbass and voted before reading), and I realized how ridiculous this thing is. There is a bunch of specific theory morning that can be done with specific mons at specific percentages, but it really comes down to two things. One is the nature of a competitive pokemon battle. Let's say player 1 has a good matchup vs player 2. That is obviously a good thing for player 1, as he can use the turn to set up, attack, heal, set hazards, etc. player two will switch to something with a good matchup, and have his turn to do the same. Now obviously, there are double switches, predictions, and volt turn (which is kinda stupid too IMO) that mess with this, but it is still the basic structure. Gengar (and all shadow taggers) break this fundamental way of battling by instead of forcing switches, forcing the pokemon at hand to deal with it. This makes the reward for a bad matchup against it far greAter than that of a bad matchup against a pokemon like heatRan (a random example). The other thing is the move pool which allows gengar to almost gaurAuntee a kill against ANY pokemon. Now, of course no set beats everything, but it can beat everything due to 3 moves, taunt, dbond, and perish song. So let's assume its a one to one trade, which is generous as gengar often takes more. Against offense, that is a draw, but against stall, the first kill is so important, as offense is fine without a member or 2, but stall needs to have the defensive core to work. Once one thing goes down, a key set of resistances is lost, and that is enough for an offensive team to break through. And I'm not saying stall teams should be 6 0 ing everyone, just that gengar makes that first kill so easy. Similarly, gengar pairs well with (and this is not name exaggeration) every sweeper in the game. Most good sweepers have only a few checks and counters, and gengar can be customized to beat any of them, and in its sacrifice allows the sweeper to sweep. If only I could take my vote back.
Ps. Note I didn't mention base 170 spatk. That is an afterthought, showing just how op it really is.