Honestly I was underwhelmed when I started using Genesect. I expected him to wreck shit like he did last gen. Instead, he's just a very good mon, who basically outclasses Scizor as an offensive bug/steel (Scizor still has defog/pursuit/knock off sets though) and fits on virtually any team. I used a shift gear set for a bit and it's cool, but with all the physically defensive Rotoms, Heatrans and priority moves running around it's hard to sweep with. Special attacking sets with u-turn seem to be the way to go, with choice scarf being the preferred item (altho ebelt and life orb are plenty viable).
Excadrill is another staple and I'm glad he's in OU. The ban last gen really deprived OU of a great spinner. Similar to Genesect, he's not gonna sweep teams, but he plays his role perfectly.
Manaphy is really good too. I don't think he will become a staple, but he's def something you have to prepare for when you build teams. He sets up really easily and hits hard. Bring him out at the right time and he's gonna take something out. Unfortunately, he's too slow to sweep, and you obviously can't run scarf or you can't tail glow.
Landorus is pretty good, not as good as the first 3 but worth consideration. He still hits stupidly hard, but there's more out there that can check him. Mandibuzz, Goodra, the blobs (who are more common now), plus the tons of faster threats and anything with strong priority like Talonflame or Azumarill will take a huge chunk from him if not outright KO him. If you need a glass cannon special attacker who's not named Genesect, Landorus is worth a shot.
Deoxys-D is great for both stall teams and offensive teams. For offense, you can easily get up rocks, status something, and if you're still alive use him as a pivot later in the game. For defensive teams, you can use him as a fast taunt with toxic and/or hazards. Those are his roles and he performs them pretty well. He's no Excadrill, but I could see him hanging around OU.
Deoxys-S is pretty meh. Support sets are really weak right now with all the defog going around. Life orb attacking sets are the saving grace, but he's still really weak to priority. I doubt this guy sees much use in OU.
Thundurus-I can still paralyze stuff. With all the priority though, it's not that great. Even the sweepers who you would really want to revenge paralyze like M-Kanga and M-Lucario have priority, so in order to paralyze them you would have to sacrifice Thundy and take a strong priority hit with your next poke (unless you get really lucky with the full paralysis). He's still a fast electric type, but if you want a pure sweeper Thundy-T hits harder, while Zapdos has better defenses. Won't see too much use in OU imo.
Tornadus-T is a mon I always liked a lot, but with that 70% accuracy on hurricane he's just not worth using unless you have a dedicated rain team. Unfortunate for a poke who was a monster last gen.