This is the Genesect I always used in DW matches:
Genesect @ Choice Scarf
Ability: Download
Nature: Rash
EVs: 52 Atk / 252 SpA / 204 Spe
- U-turn
- Thunder
- Ice Beam
- Bug Buzz / Flamethrower
Having played DW for a long time, I have no doubt that Scarf is by far the most efficient use of Genesect's abilities. Furthermore, every ScarfSect needs U-Turn; Download-boosted U-turn coupled with BoltBeam coverage is invaluable. Genesect was fantastic at creating momentum and checking threats like Tar, Politoed, Lati@s, Dragonite, etc. all at the same time. It's no exaggeration to say that Genesect was
always the key to winning - I can say with confidence that in over 90% of the matches I played against opposing ScarfSect users, the first player to lose their Genesect also lost the match. I always used Naive to allow Genesect to take priority moves a bit better. The EVs put slightly more punch in all of its attacks, and let Genesect hit 285 Speed. It was just enough to U-turn out of Chandelure and revenge DDNite - DD Haxorus wasn't common, and didn't care about tying other ScarfSect because I would never risk losing that speed tie.
I always used U-Turn/Thunder/Ice Beam, with Bug Buzz or Flamethrower in the last slot. Flamethrower was useful primarily against opposing Genesect, Ferrothorn, and Scizor, whereas I liked Bug Buzz as STAB that didn't force me to switch out, and fared better against the likes of Slowbro and Reuniclus. With Flamethrower, I would never risk the speed tie with other Genesect; Ferrothorn was usually packaged with Rain, and I generally had other resources to deal with the (less prominent) Scizor. When I built teams that had more trouble with Sun, I sometimes ran Explosion in the fourth slot to surprise Ninetales with a 250 BP, Download-boosted nuke. However, I would never consider letting Genesect die unless I was almost certain it could take out an opposing weather inducer.
An important distinction between Genesect and Scizor is that the Download boost coupled with Scarf boost and great coverage makes Genesect so much more immediately fearsome. On the other hand, especially from early to mid-game, I found myself mostly spamming +1 U-turns. Things that could stay in on Scizor - think Forretress, Skarmory, Keldeo, Thundurus, Dragonite, and even Politoed, can't risk doing the same against Genesect for fear of its special coverage moves, making racking up U-turn damage that much easier. And yes, this is pure theorymon, but standard ScarfSect handles most of this July's top 20 very well. Heatran, Terrakion, and Gengar are the most solid answers to Genesect out of the top 20.
1) Politoed - Thunder(bolt)/U-Turn
2) Scizor - Flamethrower
3) Ferrothorn - Flamethrower
4) Dragonite - Ice Beam
5) Heatran - If your opponent is smart, they will switch Heatran into Genesect 100% of the time: U-turn out every time.
6) Jirachi - U-turn/Flamethrower (if weakened, and not in rain)
7) Breloom - Ice Beam
8) Gliscor - Ice Beam
9) Tyranitar - U-turn
10) Tornadus - Thunder(bolt)/Ice Beam
11) Starmie - U-turn
12) Rotom-W - U-turn
13) Thundurus - Ice Beam (beware of Scarfed Thunder)
14) Salamence - Ice Beam (I expect Jolly DD MoxieMence will rise with Genesect's release, as it handily outspeeds ScarfSect)
15) Forretress - Flamethrower
16) Latios - U-turn (beware of Scarfed variants with HP Fire - no joke)
17) Mamoswine - U-turn
18) Terrakion - U-turn
19) Gengar - U-turn
20) Tentacruel - Thunder(bolt)