Good thread
As for Enel, I think he would make a great contribution toward the war. I doubt he could take on an Admiral or Wb, marco or joz, but I think he's capable of beating a shichibukai. He has mantra to help reading movements, can teleport (practically) and has a powerful array of movements. And to top it off he has the advantage over most of them being out at sea (water transferring the current)
We have seen how powerful his Logia is so I have no doubts about the force necessary to beat some of the toughest opponents, this would even include something on the scale of Mini Oars. And could work well against even other df users like AoKiji (element speculation)
The biggest weakness I have seen with Enel is his physical strength, and how much punishment he can take. If he can be caught then he can be beaten but reaching him would be tricky, either because of his speed or mantra. This was shown when hit by Luffys lower tier punches, they aren't that impressive compared to the current strength level we are at.
I think the only people that could stand up to him would be opponents with the ability to either beat his speed or be insulators against his electric attacks. A few examples would be Joz, and Kizaru.
As for the whole Haki debate, I wont touch on it too much, but will say that those with the abilities to use haki are likely to stand a chance at beating him. Mainly because they too could possibly read movements like Sandersonia, or could nullify the df as shown with Raliegh.
A fare amount of this is speculations but most the information given so far leads me to this conclusion.
I think everything in my post has already been said but Enel is just too interesting to avoid posting about.