I probably won't be voting this time, but I'll throw out my two cents, which are pretty easy to disagree with.
I feel that... None of these suspects deserve a ban. None of them.
Mega Lucario is good at setting up, has great typing, stats that rival Arceus, yadda yadda. Mega Lucario is also unable to conquer opposing fighting types, especially those with Mach Punch. Conkeldurr kills it with Drain Punch, obviously, and it can beat it also with two Mach Punches. Lucario does 71.4 - 84.5% damage to a 252 HP Conkeldurr with Close Combat, but that lowers its defense, allowing Mach Punch to do 90.3 - 107.4% damage back to Lucario, which is a 43.8% chance of a OHKO (56.3% after rocks). If Conkeldurr comes in on the set-up, it can kill Mega Lucario after a bit of residual damage. Mega-Heracross, the mega that I've been supporting this generation, also can kill Lucario with ease. After a swords dance, Close Combat cannot kill Mega-Heracross, while Close Combat (or even Arm Thrust) gets the OHKO without any set-up. What else can kill Mega Lucario? Scarfed Pokemon. Scarfed Heatran, for example, OHKOs Lucario using Lava Plume without any investments in special attack. Assuming it switches in on any move other than Close Combat, Heatran will get the kill. Gourgeist, a Pokemon I like to use, can switch in on Close Combat or Extreme Speed and use Will-O-Wisp. Ice Punch isn't even a OHKO, it merely 3HKOs.
Lucario is tough, Lucario is very difficult to best. There are ways, though, and teams can easily adapt to this. There are plenty of methods to beating it.
What I'm saying is, Lucario is not
unstoppable. That means it should be okay in OU. Blissey completely walls special attackers, but since there are physical attackers, it is not
unstoppable. Ubers are unstoppable in OU, which is why they are not in OU. I'm really breaking this down more than I need to, but I want to ensure that my point is received.
Deoxys-S? I have not had problems with Deoxys-S. It is awesome at setting up hazards, but there are other Pokemon that are great at removing them. Defog is very popular, so threats are much more easily removed. Deoxys-S cannot help but be set-up on by other Pokemon, methinks. There's the attacking version of it, but it isn't able to defeat other sweepers. Deoxys-S has the ability to set up hazards, easily removed, or get the first hit with a diverse moveset. What I find most threatening is trying to guess what moves it will have, but it only takes one or two moves to find out completely. Brave Bird, on a jolly banded Talonflame, has a 81.3% chance at a OHKO. I find that incredible. Deoxys-S cannot usually survive the most used priority move in the metagame. Factor in rocks and it
never can survive it. If you lead with something able to OHKO the Deoxys-S, then I think it won't be able to set up. On Aegislash (Blade form), Shadow Sneak barely doesn't OHKO. However, nothing that Deoxys-S has can kill this Aegislash back. Deoxys-S is not bulky or powerful enough to function as a sweeper.
That is not what Deoxys-S is, though, is it? He's a revenge killer, or a sweeper. I believe that a strategic player would use their Pokemon in a way to conserve what could kill Deoxys-S, which only allows it to do a decent amount of damage to a single Pokemon, which kills the Deoxys-S in that turn. Or, if it's a defensive team, it would either taunt, defog, or kill using a couple attacks without much of a problem.
Deoxys-S is not unstoppable. It is very stoppable.
Genesect is the only one of these suspects that I'm guilty of using. I've been using an expert belt set, and I've been impressed with its sweeping and walling abilities (By walling, I mean switching in on a fairy and requiring it to switch). Genesect is incredibly diverse and powerful, and has a difficult type. There are very few flaws in this cannon-bug, but here they are, in list-form:
~4x weak to fire
~Not incredibly fast
~Can be walled, but difficult to predict
~Only functions as offensive
~No priority
So, to narrow it down, Genesect can only fight. That is its only ability. Deoxys-S can set up hazards, and Mega Lucario can use priority or functionally use substitutes. Genesect's only way to be faster than the average foe is via choice scarf. Otherwise, it has 99 base speed, which is possibly the most unfortunate speed in the game (Except that it's a nudge faster than Hydreigon). How can you live an attack by him and kill it in one shot, though, before it kills you? Well, fire types have an easy time dealing with Genesect, as it never has anything super-effective, and relies on moves that could be possibly beneficial to the opponent. For example, Heatran. Heatran is arguably the best counter to Genesect. Very few carry fighting moves, let alone anything stronger against it than Thunderbolt. You can switch in on Flamethrower and sweep with Lava Plume after, which is a move that nothing likes switching into besides another Heatran. Blissey can wall some forms of Genesect and paralyze it, and even set up a wish or SR. Water types usually have an okay time with Genesect, and can kill it with Hydro Pump or Surf. There's a few ways to deal with it, but obviously, determining the version of Genesect is key to defeating it.
Genesect is a stellar Pokemon, as are the other two suspects. I don't see why it should be banned, though, even if it is very useful. It fits in any team like a puzzle piece, and it can sweep teams with no set up, but it, like the others, is stoppable. To build a team is to factor in all major threats and focus on how to deal with them. If a player isn't able to use a Heatran, then I believe that is the fault of the player, not Genesect.
I haven't played here for a long time, so my opinion may be a bit strange, but I have a ~1500 rank on the OU ladder, so I think I can at least share what I think.
[EDIT]: Thought I'd post this just so that you know I'm not lying: