Grab a Modest for a RP set
I'm going to have to agree with dragonuser in his proclamation that LO RP Genesect is the best Genesect set, despite what my poll choice (EBelt RP Genesect) says. It rips through almost all of the OU metagame and can force the hand of many offensive teams once it reaches +2, granting its team a huge advantage. Heck, it can be used to deal damage early on in the game without needing to use Agility, thanks to Download. Also, this may sound weird and (for some of you) noobish, but I've found success in pairing RP Genesect with RP Sheer Force Landorus. Most teams I've faced down on the ladder simply cannot deal with both of them together, and this effect is amplified by people relying on Specially Defensive Rotom-W to check pretty much EVERY specially-based threat in OU and Scarf Genesect as their only revenge killer. Now, EBelt RP Genesect is pretty cool because it can feign a Scarf set, demolish walls like the pure EBelt set, and sweep on a moment's notice with one boost, just like its LO counterpart. Obviously, it won't be as powerful, but this allows you to play mindgames with your opponent, possibly snagging you the game if played right. However, it lacks the consistent power that LO brings, giving it a miniscule amount of inconsistency.
Personally, I don't exactly see why Scarf Genesect is so popular. It's bait for DragMag teams, and it doesn't pack the punch that the Choice Band and Expert Belt sets are known for having. Sure, it can grant you momentum from Turn 1, but it gives Lead Terrakion, Deoxys-D, and Normal Gem Mew a free turn to set up Stealth Rock (i.e. barring the effect of crits). However, don't take that statement as me saying that Scarf Genesect is a bad set. It's quite good at securing momentum, which is important for any offensive team, with its lightning-quick U-turn, allowing it to work well with almost every team archetype, ranging from weatherless VoltTurn (Eclypse's Volt-Turn's Return) to Sun offense (Lavos Spawn's Simulation of a Drought). In fact, Scarf Genesect (alongside Dugtrio) is responsible for the introduction of Shed Shell Heatran and a small resurgence in Scarf Heatran's popularity. Scarf Genesect's ubiquity caused the popularity of +Spe DD Salamence, which Genesect cannot check, to rise, and DD Dragonite, which Genesect can check, to fall, among other changes. Of course, the issues of Genesect being DragMag bait, its lack of power, and disadvantages if used as a lead cause me to regard it as an inferior set.
Now, a small quip on my preferred EV spreads. Should I be running a Scarf Genesect, I personally prefer an EV spread of 112 Atk / 144 SpA / 252 Spe. While it forces a compromise with the amount of damage done, at +1, Genesect can OHKO 252 HP / 0 Def Latias after just Stealth Rock damage. That is huge for offensive teams that employ Sheer Force Landorus, which Latias walls ad infinitum. If maximizing Speed isn't of concern to you, then you can go with Funkasaurus's EV spread of 64 Atk / 252 SpA / 192 Spe, which gives Genesect's attacks a bit more sting, while outpacing +1 Dragonite and Gyarados. However, if your team doesn't exactly care about Latias, then you can go with the standard EV spread given on Genesect's analysis. For the Rock Polish sets, I prefer the standard EV spreads, but I've been tinkering with a bulkier Genesect spread to grant it a bit more survivability.
tl;dr: LO / EBelt RP Genesect > Scarf Genesect. Small paragraph dedicated to preferred EV spreads.
Seconding this. Back when I ran a balanced team, SpD Rotom-W was super clutch for checking a bunch of annoying threats but I was getting tired of getting my ass handed to me when I missed a Hydro Pump vs Genesect, or losing out to Giga Drain Genesect, etc so I started running T-wave and it alleviated most of my problems. Nearly all of the Pokemon that commonly switch into Rotom-W (Latis, Dragons and even Deoxys-D) HATE Thunder Wave. Not burning Ferrothorns isn't that big of a deal, and the paralysis can actually come in handy at times.This is why I prefer T-Wave on my specially-defensive Rotom-W, since all the mons that it's supposed to check (namely QD Volcarona, Genesect, Tornadus-T) and Pokemon that it lures (Lati@s) are crippled by paralysis more than burn.
Plus paralysis is such an amazing utility; even if you end up paralyzing Ferrothorn it's okay, because 25% chance of immobility cuts down in its spike-stacking capabilities. fp wins you games - Wisp misses loses games.