Kidd has been presented as Luffy's top rival among the supernovas. He is definitely not as weak as some people seem to think. My last paragraph describes the potential of his ability. He'll likely be a Yonkou level pirate of Luffy's era, as I suspect Law will as well. I completely agree with dice that Oda didn't put the three of them in a trio for nothing.
As many people have pointed out with the Faraday talk, electricity and magnetism go hand in hand. Fluctuations in one cause fluctuations in the other. Therefor, Kidd has a very good chance against enel. If Kidd located all the gold, he could use it for additional defense. Honestly, the winner of a fight between electricity and magnetism will be the one that understands the Faraday relationship better, but I'd guess that both have mastered it and it would be a push. To be honest, this fight could very well happen in the future, and it would likely come down to their non DF combat abilities.
It seems to me that Kidd would have an advantage fighting Moria, since Moria seems to have a lot of metal around and relies on it. Moria probably has a better chance against Luffy (who needed significant outside help to win). Any swordsman would be a joke to Kidd as well. Lucci could probably mess up Kidd pretty bad, since Kidd's ability wouldn't be worth much against his tekkai. As for Crocodile, I have no idea, he's like a glass cannon. I guess I'd put my money on the croc, hesitantly.
Either of the admirals we've seen would annihilate Kidd, just like they would Luffy at the moment. Kuma, however, would get torn to pieces. A man made of metal vs a man who can freely control metal? That's not fair. Lastly, I don't think Kidd would fair very well in ID because most of the stuff there is seastone, not metal. He'd certainly have no answer for Magellan's hydra.
Against the SH crew, I think Luffy would likely beat Kidd in a tough fight. Zoro, Usopp, Chopper, Franky, and Brook would all stand basically no chance against his DF power. I know there are plenty of Zoro fans here, but let's not forget what a rustman did to Zoro, even though he was weak enough to get defeated by Usopp. Sanji, Robin, and Nami would all give Kidd an interesting fight. Sanji and Robin for obvious reasons, Nami because she should understand magnetism as well as anyone. Nami would lose, but she could put up a fight (similar to how she fended off Enel for a short time when nobody else could).
The last part of this essay, and the most important, is about Kidd's ability itself, it's retardedly powerful for someone who understands physics. Using strong fluctuation magnetic fields and inducing a current, he could easily melt metal and freely control it. Then by stabbing someone with metal, he'd be able to transfer heat or electric shock by manipulating eddy currents within the metal. A magnetman is be one of the most powerful paramecia types I can imagine, as long as there are conductive materials around. This guy would basically be a beefed up version of the third kazekage from Naruto once he mastered his power, and that's no joke.