Apologies if some of this has been said already, but I think it's important to cover all my points in one post rather than having to refer back to other things.
I've never really liked the term "broken", both the word itself and the meaning. I just don't think it's possible to construct an objective standard, and thus clearly these judgements will be highly subjective. In my opinion, whether or not a pokemon should be banned is not entirely to do with how good it is. I think there are some things that are healthy for the metagame, and other things that are not. Primarily, I don't like to see a metagame where results are based on luck, in one form or another. This includes team matchup issues, to an extent (although some teams are of course, just bad). We've got to a point where, unless we make a whole slew of bans, it's never going to be possible to counter everything with one team. There are too many scary offensive threats, particularly set up sweepers, out there. Now I notice that of the three suspects that have been proposed, one of them is a frightening sweeper, while the other two are primarily used as revenge killers. Revenge killers are the way that you get around all of the scary threats, the way you can cover your bases, such that you do stand a chance against the majority of attacking threats and the majority of attacking teams. This reduces the impact of luck on the metagame, so I think that in general revenge killers are healthy for the metagame.
One thing our current list of suspects has is that is that they all have very few good counters. We've come to accept that there will be pokemon with very few counters in OU for some time now, for at least a generation. The key thing to realise is that it is not OK for sweepers to have no counters. Essentially, if a good sweeper beats its counter, then it has a good chance of killing the rest of your team too. If you can't counter something that is not good at sweeping, then you lose a pokemon. This is, of course, a big deal, but unless the pokemon in question finds it easy to do this multiple times, especially if it can switch in, this is not "broken" in and of itself. Lucario, specifically, is a potent sweeper. If it sets up a boost and its counter goes down, it's pretty much toast. You have no option but to counter Lucario somehow, and this is really quite difficult, as many people in this thread have no doubt already shown. Lucarionite deserves to be banned for sure.
However, the other two pokemon are more questionable as suspects. I'll discuss Deoxys-S first, since I'm rather more sure about where it belongs. By and large, Deoxys-S has two sets, the hazard setter and the Life Orb sweeper; I don't think any others are worthy of consideration (perhaps a hybrid of the two, I suppose). I think the Life Orb sweeper set is far and away the better of the two. The hazard setter is not all that much better at its job than its competitors, and the effectiveness of its hazards are more questionable than ever. Yes, I still think SR is the best move in the game, but setting up spikes in addition I'm less convinced by. Setting up SR on its own is almost always a net gain, since it takes as many turns to remove it as it took you to set it up, but if you suicide your hazards setter for SR and a layer of spikes, you've really lost out on a lot if they are removed. I understand that this set would probably be run on a very offensive team, but I don't really believe that you wouldn't be able to find one turn to remove the hazards. If you really can't find a single free turn throughout the entire game, which I find quite hard to believe, does this not speak more of deoxys's teammates than of deoxys itself? Deoxys's whole team would have to be horrifically overpowered. In reality, when I don't defog the hazards away in such a situation, it's not because I couldn't, but because I chose not to. If I had to choose between removing two layers of hazards on my side of the field, and getting a KO, I would quite often choose the KO. I also think spikes are less effective because more flying pokemon are being used, which are immune. As for how effective deo-s is at setting up hazards, I find it less effective than deoxys-d. There is so much priority right now that I think most teams should be able to limit deoxys-s to stealth rock, assuming that it leads. Aegislash, in particular, can deal with deoxys-s quite easily, as can talonflame. If deoxys-s isn't leading, it might well get more, but in that situation I think a deo-d would do even better. Also, if you only want SR, Garchomp and Terrrakion are really great, as they are useful both offensively and defensively, in addition to their support role.
The Life Orb set is, as people have said, reasonably hard to counter. Aegislash, mega mawile, along with a few other steels and some assault vest users are your best bets. But as I said before, having few counters doesn't really make a pokemon overpowered unless it is threatening to sweep your team. Contrary to what people have said, deo-s is not a good cleaner. It has two stat-dropping moves that it relies heavily upon, it very often 2HKOes things rather than OHKOing them, and apart from anything else, it's effectively not fast enough because it usually lacks priority. I've used deo-s alongside priority based cleaners like CB Genesect, Talonflame, Lucario and Pinsir, and I find that these clean far more often than deo-s would, even without setup. What LO deoxys-s provides is great revenge killing, which for me is a good thing, and some mid-game wallbreaking power. It isn't really able to switch in at all, so I don't feel that it is able to come in and kill something enough times to be considered overpowered. edit: I also forgot to mention that the deo-s user often has to guess correctly to get around its would-be counters.
As for genesect, I really don't know. I really appreciate the revenge killing capabilities provided by the scarf and band (espeed) sets, and I do think genesect is quite counterable, excluding U-turn. However, there is just something inherently broken about U-turn; in the typical situation we consider to establish if a pokemon counters another, the genesect team almost always comes out on top. Basically any pokemon with U-turn is uncounterable; all it has to do is be able to switch in, and force something out. Genesect is perhaps the most able to do this of any u-turner we've seen, not to mention that its U-turns actually do substantial damage. Overall though, I don't know. I've had far less trouble with this pokemon than previously. Part of this is because Aegislash is a wonderful switchin. The sweeping sets, which posed a lot of the problem last gen, don't seem to be too common, and I've seen very little trapping used alongside genesect (although I always though the trapping was the side of this worthy of banning). What I see is mostly revenge killers, which as I've said, are a positive influence. Perhaps this is just that people on the ladder haven't worked everything out yet, I don't know. I just don't see too much of a problem at the moment, even if there might be one in the future.