Part of my perspective on this is that we really can't be expected to face every threat in the game equally well with simply a single typing, and that we sort of have to consider the fact that what we pick in these polls has to be able to point us in a direction where we don't have several bad weaknesses to cover for, and eventually bring us to some contrived ability to work out some of those weaknesses. For these reasons, I think that neutrality is the best thing to be going for in this poll, and the choices here reflect a balance between "strong resists" and "lack of bad weaknesses". My choice also reflects my opinion on where we ought to be going on this spectrum.
I voted in this poll for Electric. The reasoning behind this is that Electric is arguably the "ultimate" neutral typing other than Normal, in that it only has a single bad weakness (that is not difficult to negate or neutralize) and a handful of resists in addition to being hit neutral by most attacks. This gives us a good base to work with that doesn't effectively compromise or bind us in a situation where we have a bunch of strong and useful resistances, but multiple bad weaknesses to work out with another typing that simply doesn't cover everything. In simplest terms, Electric gives us a typing that could effectively work equally well against just about everything, and it has very minimal drawbacks that can pretty much be addressed by a single thing (whether it's typing or ability).
Let's look at the other choices (as far as I'm concerned, none of them are bad choices, and nothing that was allowed into this poll is going to come close to ruining the Pokemon, but I prefer some to others):
Fighting is similar to Electric in that it has very few weaknesses, only two that aren't too commonplace in standard OU, and takes mostly neutral hits. In addition, it has three resistances that are pretty useful, those being Rock, Bug and Dark. These three types generally have some very powerful physical users behind them, particularly Scizor and Tyranitar, some of the most difficult Pokemon to switch into, and with this general advantage on the physical side, this would probably provide us with considerable leeway to focus primarily on Special bulk for taking strong attacks like Draco Meteor from Salamence. Generally speaking, I think this is one of the stronger routes we could be taking. However, I chose Electric largely because defensive Fighting-types have been tackled many times before, and TWICE in CAP (one of them was also supposed to be a decentralizer) and I feel that Electric has more to offer from a learning standpoint.
Water has a few nice perks going for it in terms of resistances, particularly Fire and Water (Ice is rarely ever STAB, so it's not that difficult to wall in general), and it's a neutral defensive typing for the most part. For these reasons, it's also a fine choice. Its two weaknesses, however, aren't particularly fun weaknesses to have when one is trying to tailor something to respond to specific threats. An Electric weakness really needs to be fixed, considering the number of users, many of them STAB. Grass is also a deceptively dangerous type to be weak to, as the few Grass-type moves in the metagame tend to be pretty strong, most of them being backed up by STAB, though this weakness is probably not the worst concern. In addition, there are already plenty of bulky Waters in the metagame, and we would need to make it a point to differentiate from the standard bulky Water switch-in utility.
Steel has a boatload of inherent resists, the most prominent of which being Dragon. In fact, the Dragon resist itself is probably one major perk that Steel has above the other typings, on the basis that Dragon sports the most powerful moves in the game from a general standpoint, and that this basically makes it much easier to respond to that immediately. However, this comes at a cost of having three horrible weaknesses in the form of Ground, Fire and Fighting. These three typings are generally thought to have pretty good coverage with one another, to the point where we probably wouldn't be able to cover them with another typing/ability combo. Ground is generally a strong type that you don't want to be weak to, but it's not the worst thing in the world because it's not that difficult to circumvent and most users of it don't get STAB on it or are defensive Pokemon (except for Mamoswine and Flygon really). On the other hand, you simply CAN'T be weak to types like Fire and Fighting and be able to stand up to threats like Heatran, Infernape and Lucario. While the Dragon resist is important to have, I want people who are voting for Steel to seriously consider how exactly we're going to deal with these problems when they arise later, which they will have to.
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These are my general opinions of the typings in the poll. Again, I don't think that anything in the poll is a terrible choice, and pretty much any of them is guaranteed to be an interesting and suitable type for exploring this concept. This post is to show you where I stand, and to give a good idea of how each of these individual typings will affect the course of our project, hypothetically.