I'm really glad to see Mega Lucario finally up for suspect. I was actually fairly surprised it wasn't insta-banned considering how ridiculously flexible it is combined with brutal 145/140/112 offensive stats. I don't believe I can contribute anything more than what has already been said, but I'll give my reasons as to why I believe it should be banned.
Unpredictability: The fact Mega Lucario has three extremely viable sets, all of which defeat the few counters the other sets respectively have, means you nearly always lose a mon if you guess incorrectly (I'd say predict incorrectly, but you can only make an estimated guess based on the opponents other 5 mons until Mega Lucario has revealed a move). If I send in Goodra, physical Luke wrecks it. If I send in Landorus-T, I'm promptly destroyed by a special variant.
Sheer Power: Plenty of calcs in this thread have demonstrated how a +2 adaptability close combat wrecks physically defensive counters that RESIST it. That is too much for the metagame in my opinion. Fantastic offensive stats combined with higher than average speed means Mega Lucario is far tougher to revenge than the average mon. With brilliant +2 boosting moves on either side of the spectrum and a decent defensive typing, it is far too easy for Mega Lucario to capitalise on a Pokemon it threatens out and destroy the majority of the metagame in response.
For the above reasons, I'm voting to
ban Mega Lucario.
As for Deoxys-S, I feel it deserves to be in OU for the following reasons:
Typing and bulk: Poor bulk combined with an abysmal typing means Deoxys-S won't be living many hits. 50 base HP really hinders decent defenses, and everybody knows how terrible psychic typing is, as it offers little to no defensive synergy. It's susceptible to every form of entry hazard and all status conditions, further hindering its ability to survive for a long time. It needs a life orb to deal decent damage too, which further aggravates its circumstances.
Average offensive stats: I agree with those that say the life orb variant is the most threatening, but as many calcs in this thread have demonstrated, and from personal experience, Deoxys-S lacks the power to KO a lot of important threats (if people disagree with me here I'm more than happy to provide calcs, though I'm so late to the thread that there's plenty of evidence already). You definitely have to be careful to make sure Deoxys-S can't finish your team off after a long battle, but that goes for any decent cleaner.
Defog buff + the current meta: With a new way to remove entry hazards, the lead set has lost effectiveness. Deoxys-S is absolutely fantastic at setting up stealth rocks, but often that's all it does. Taunt can be seen coming a mile away and provides an opportunity to slam Deoxys-S for heavy damage. Defog means Deoxys-S often died in vain, and I don't believe it's worth losing a mon to secure stealth rocks in a metagame where they're fairly easy to dispose of, but I know people may disagree with that opinion. Furthermore, the shift to bulky offense is something Deoxys-S really doesn't appreciate, as it excels at destroying frail speedy threats, not bulky slow ones.
For the above reasons, I'd like to see
Deoxys-S remain in OU.
Genesect I have been most on the fence about, just because it isn't as obviously broken as Mega Lucario, however, I do feel it's ban worthy for the following reasons:
Momentum: Nothing generates momentum as easily as Genesect does. Download allows Genesect to prey on the weakest defense stat and threaten accordingly with fantastic coverage. This often generates a switch which Genesect capitalises on with (potentially) the most powerful STAB u-turn in the metagame, which packs a punch whilst maintaining momentum.
Risk vs. reward: In playing against Genesect, the reward for predicting correctly is minimal whereas the risk is often major. In most scenarios, you take a powerful hit, lose momentum, and deal minor damage as a threat will come in that handles you appropriately. The pressure Genesect causes is immense, as often you risk either losing a mon to a powerful super effective move, or losing momentum whilst achieving nothing. Genesect feels like an easy win button that foregoes prediction for u-turn spam.
Hit and run: Genesect often wears down its counters with u-turn spam. Saying Rotom-H and Heatran can handle it isn't a viable argument when Genesect is chipping away at them and bringing in a powerful threat to wreck them, forcing them out before they can do anything. Genesect isn't going to be sweeping teams on its own nor is it going to be beating its counters, but it's going to generate a response to eliminate those counters without breaking a sweat.
For the above reasons, I believe
Genesect should be banned.