I'll just repeat myself again.
Nowhere in the concept is it written that this CAPmon has to benefit Togekiss without any gain for itself. This is not a "reversal of roles", it is mutual symbiosis.
Leading on from that, this is the way that I believe that the concept chould manifest itself as a CAP. This is merely an opinion, so doubtless irrelevant to many, so feel free to skip this if you would like. Alternatively, read it so that you know where I am coming from in my arguments. I believe, as I have stated, that this CAPmon has to have both offensive and defensive synergy with Togekiss. This much is quite obvious. Togekiss is an intriguing case in that it provides to a great extent what we are looking for in CAP11. The ability to constitute both an offensive and a defensive threat, and to support its teammate(s), is exactly what I am looking for in CAP11. To provide this, the typing has to accomodate both offensive and defensive strategies, regardless of what the overall intention is. I say "accomodate" not "focus on" as we have a clearly definable ibject in mind, but all possibilities may be considered. Constant switching requires defensive synergy, or the combination is worn down quickly. It is essential that this combination provides a threat to teams which is clearly identifiable, which is why offensive synergy is required.
However, one thing we cannot afford is to be complacent when considering which road to take. This means that conventional theorymon, as it is called, is limited in what it can achieve for us. A prime example of this is the Ghost-type argument. In theory, luring Shadow Balls for Togekiss to come in on is a good idea, but in practice, we have to consider so many other factors than this. The types that lure Shadow Ball are Psychic and Ghost, both of which are far more likely to lure Dark-type moves in OU, if I'm honest, as well as the fact that these two types don't lure any other types of moves, and don't have any offensive or defensive synergy besides that, AND the two Pokemon who use Shadow Ball as their main STAB are Rotom-A and Gengar. Neither of whom Togekiss really wants to be in against. Similarly, we cannot simply say that our typing should lure Grass or Bug attacks because those Pokemon that use it as STAB can all pretty much marmalise it with a different attack. So, we have to think of the effect that will take place in a competitive context, not simply that which appears on paper.
So, consider this. The metagame will change when this new CAPmon is introduced - you can absolutely count on that. So, what we want to happen when this new CAP is introduced is the reaction of the metagame to an offensive threat, as per precedent. Hence, what we want to happen is the shift or orientation of the metagame towards Pokemon that Togekiss can deal with but other Pokemon can't. So, to deal with this threat, the counters should ideally be bulky Ground-types such as Swampert and Hippowdon, or weaker offensive Water-types such as Vaporeon. Hence, a metagame shift that benefits Togekiss. Zapdos dislikes Swampert and co, while Gyarados dislikes Vaporeon and Celebi. But Togekiss deeply appreciates this state of affairs, hence why I feel that this is the effect we should be going for.
/end speech. Now, some reasoning as to why certain types just don't make the cut:
Psychic hinges almost in its entirety around being able to lure in foes such as Tyranitar and Scizor and OHKO. Aside from that it does nothing. You may say that it can lure in Ghost-type moves, but realistically, what Ghost-type does Togekiss stand a chance of beating? Rotom-A is one of the best Togekiss counters around, and Gengar isn't going to switch in on a Psychic-type. And even if it did, Togekiss can't really afford to switch in due to Thunderbolt:
Unboosted Timid Gengar Thunderbolt vs. 252 HP Togekiss: 49.2% - 58.3%
And even then, most Gengar run Substitute nowadays. There are infinite things in the game that benefit from the removal of Scizor and whatnot - Togekiss is not exactly the greatest Pokemon to take advantage of that. It really isn't going to help that much - especially considering the wealth of other options at our disposal. Conversely, it just gets worse - If you can Pursuit-trap with the right Pokemon, the combination is ruined. There really isn't any point to a combination if you can't abuse it, and the sheer increase to possibility of disappearing so early on in the battle is not exactly enticing. Psychic gives us no defensive synergy, no offensive synergy, and just seems like a gimmick option. If our collective desire to "try something new" is stronger than our competitive reasoning, then this is probably the best option that there is. Otherwise I don't see the attraction. There is no symbiosis. There is no synergy, and the desired metagame effect does not happen. Instead of making Ghosts and Tyranitar and Scizor less common, it encourages them to be more common. This state of affairs does not benefit Togekiss.
As far as the Ground-type goes, some of you are really, really underestimating the effect of the double weakness to Ice-type moves here. If the combination is as effective as the concept would lead us to believe, I can almost guarantee that every team will begin restocking on Ice-type moves to break it (of course, if it isn't as effective, nobody will bother and likely the concept will have failed). Mamo goes up, Starmie goes up, all bulky Waters with Ice Beam. The concept can still work, but certainly not as effectively, especially if it can be broken up by a single move type that actually has so much utility already in this metagame. One move on many Pokemon that easily breaks this combination is a bad thing, a very bad thing. Then we come to the last part - the part where the metagame is affected by the inclusion of a new Ground-type. Flying-types come to mind as increasing, which is definately not what Togekiss wants, never mind the fact that other Pokemon can use this sort of support much more effectively than Togekiss. Gyarados and Zapdos spring to mind.
And as for Water and Steel. Where on earth did they come from? I can see what some people are thinking, as they do partner well with Togekiss to a certain extent. But that's exactly the point, isn't it? These two types are extremely popular because they pair well with just about everything. Almost every team can use a bulky Water or Steel because they patch up so many very important weaknesses. Togekiss's improvement from this addition is basically null, as so many other things partner with them better. With regards to the metagame change brought about by the inclusion of CAP11, a strong Water-type will most likely lower the effect of Flying-types and Dragons in general, while at the same time increasing the use of Grass-types... but this means pretty much nothing to Togekiss, as the metagame change will more or less pass it over. The same is true for another Steel-type.
So, I've briefly explained why those four types are impractical and not really the best choices. So, now the popular types that I favour, namely Rock, Electric, and Poison:
They all pretty much do the same thing, which is to orientate the metagame around stopping assaults from those sorts of Pokemon. All three can increase the usage of bulky Ground-types and, minus Electric, increase the usage of bulky Water-types as well. This relies heavily on the "counter mentality" remaining true in this metagame, but I think that this is not too unlikely an assumption to make in terms of the reaction we want to create. Poison is an interesting case in that it creates a more defensive situation compared to the other, but going on what Fuzznip has said about this being a primarily offensive partnership, I would have to select
Rock and
Electric above all others.
EDIT: @Raikaria:
Yes, Steel STAB isn't gonna get people jumping for joy, but the concept is to help the OU pokemon sweep. WE ARE NOT BUILDING A SWEEPER. If this thing can sweep, we've failed in our concept, and made a pokemon that benifits from Togekiss, more than Togekiss benifits from it. Electric and Ground are both capable of Sweeping with their good stabs. Steels, less so. Lucario dosen't use Steel STAB, nor does Emploeon, nor does CM Jirachi, and Metagross just has absurd attack, a chance to raise it, immunity to Intimidate, and even then needs Agilty to really sweep.
When did we decide we were not building a sweeper? As far as I can see, the concept was "Perfect Mate" not "Perfect Supporter". Nowhere in the concept is it written that this CAPmon has to benefit Togekiss without any gain for itself. This is not a "reversal of roles", it is mutual symbiosis. This is necessary for the CAP to succeed, otherwise there is literally no benefit to using Togekiss over any other special Flying-type, or even special sweeper full stop. The CAPmon IS supposed to benefit from Togekiss AND Togekiss is meant to benefit from it. Mates look out for each other. The combination has to be cohesive or there is no competitive value. I'm sorry to put it bluntly, but it seems that your opinion does not seem to match the concept at all.