Frankly, I think Genesect is a broken mon to the core. Genesect has no true "counter". Of course, you could say something like Heatran is a counter, but realistically when can you get it in for free without Genesect just U-turn'ing out into their Landorus-T, Dugtrio, or Garchomp, or etc? That's the thing about Genesect- the sheer amount of momentum it swings in your favor for little to no cost for the user is enormous. Obviously there are other pokemon that can do this, but none exhibit the same versatility as Genesect does. It hits hard on either spectrum, has access to priority, setup, and free Download boosts on switch-in.
It's just an incredibly overwhelming pokemon every time it drops. You literally have no idea what Genesect is going to do unless you've seen the team before. The thing is just disgustingly strong, and it's a huge problem to the health of the meta. Honestly, this exact thing happens every single time a new generation rotates around, and I'm kind of sick of the purple piece of shit and the process of disinfecting the meta of it every time. I really hope that the guys in charge of banlists every new generation put it in the same pile as Deoxys, because I'm not convinced it will ever been a balanced mon, based on previous trends. (Sorry if that comes off as rude, but Genesect is literally one of the worst cases of power creep in pokemon, I view it as a very unhealthy presence and I hate the effect it has on team building and tournament play. )
I'm not even going to mention Tapu Lele, because there's honestly nothing new to say on that thing. I also fully believe that Pheromosa and Greninja (Kinda split on if both or just protean) will warrant a suspect test in the future, but I think Genesect is a more pressing matter.