So, What can actually reliable deal with this guy? My list consists of:
Bulky Rotom-H,
Heatran
Vaporeon, it is shaky, but with a spread of 252HP/224Def/32S.Def/ with a bold nature nets it an attack boost, to which point thunderbolt is only a 3HKO. +1 U-turn does even less (with a naive nature, mind you) You can even scout it's move with protect. A bonus is that with a naive nature, Vaporeon can 2HKO after Stealth Rock. You need Surf unfortunately to guarantee the 2HKO, but it's there. It can't come in a +1 Thunderbolt obviously.
Chansey, +1 U-turn doesn't even come close to 2HKOing it. Blissey fares decently, being 3HKO'd only by a +1 U-turn. Both of them often carry protect, which is a bonus.
Terrakion in Sand is also a good check. You might have to EV it so it gets an attack boost though.
Unfortunately, Gengar is 2HKO'd by anything that isn't U-turn or Bug Buzz.
Any others I'm missing?
Volcarona is the best switch-in to Genesect if Rocks aren't up, thanks to its ability. As a Genesect user, you have to be careful of spamming U-turn around Volcarona, because the 30% burn chance from its Flame Body seriously nerfs U-turn, your most important move.
Ninetales can take anything Genesect throws at it, although it won't enjoy switching into SR repeatedly. It has the added bonus of eliminating Genesect's preferred Rain.
Gastrodon with Defense investment handles Genesect reasonably well, barring Download-boosted Bug Buzz.
Infernape has a 4x resistance to U-turn, and Genesect's best move against it is Thunderbolt. Scarfed variants can outspeed and threaten to OHKO with Fire move of choice.
Toxicroak also has a 4x Bug resistance, and can threaten to set up Sub/Bulk Up/SD while Genesect switches out.
Tentacruel resists everything Genesect has, besides Thunderbolt, and won't be 2HKOed thanks to its excellent Special Defense. It wishes it had more reliable recovery, but Genesect is often paired with Rain, activating Tentacruel's Rain Dish - just beware of Thunder.
Jirachi with Special Defense investment in Rain doesn't like taking repeated U-turns, but can tank Genesect's Special moves with ease.
Metagross is rare, but its great Defense and Steel-typing allow it to take on Genesect in Rain. It lacks recovery, but deserves a thought if your team also has troubles against Tornadus-T and/or Lati@s.