Wow this is a fun thread that we haven't had in a long time. I hope no one comes in and locks it again. Let's keep this civil! I'll only include people who have shown their powers. to lay some groundwork, these are in terms of Grand Line, not in terms of the whole world, which would include those pirates in the "Blues". Please have a look! I thought through them seriously!
My god (not to be sacrilegious or anything): Aokiji, Mihawk, Blackbeard
Frankly speaking, IMO, I don't think anyone else deserves to be in that tier. seriously. Not even Enel. I mean, when you think of "Godly", I'd pretty much look at something quite invincible to most. Mihawk is a "villain" if you consider that he is Zoro's aim and was once fought a Mugiwara pirate.
Broken: Enel, Lucci, Moria, Lafitte, Donquixote Doflamingo, Bartholomew Kuma. Possibly Crocodile, Smoker
Some may feel that Enel is underestimated. I disagree. He is powerful, but Oda himself revealed that there are bigger fish on (not "in") the oceans. Lucci deserves to be here; otherwise there'd be no point to Kuma saying "who would have guessed that Lucci would be defeated?" . Lafitte was added because of his feat in Mariejou: he slipped in unnoticed, unharmed, with tap shoes. Truthfully speaking, Doflamingo and Kuma are there because of their bounties; I mean, if Luffy and gang brought down Enies Lobby and gained 300million, it sure makes you wonder what these guys must have done. Crocodile and Smoker are on "maybe" because, frankly, I dont think they measure up to Luffy's current enemies.
Hella strong: Burgeiss, Van Auger, Ohm, Kaku. Possibly Jyabura, Mr 1.
These are self-explanatory. Jyabura's on "possibly" because even though his douriki is similar to Kaku's, Kaku uses a 4 sword style that makes him way superior. Mr 1 is also a "possibly because I don't think he's as strong as the rest here. there are fewer people here because many just don't seem to measure up to "hella strong".
strong: The rest of CP9 except Spandam, Mr 2 Bon Clay, the rest of the 4 priests in Sky Island except Gedatsu and Ohm. Possibly Foxy, Bellamy.
I don't think there'd be much contention here. Yes CP9 agents are weaker than Mr 1, besides 3 of them. Foxy is "possibly" because his method is just cheap, and not exactly "strong", especially once you get a mirror. Enel could get round his powers even when Luffy was directly immune to it, with Mantra and everything, but not Foxy. Bellamy was destroyed in one Luffy, unstretched, if angered, punch. He's more showy than powerful.
Above average: The rest of the Officer agents, Tashigi, Arlong, Sarquiss, Don Krieg, Nero. Possibly Kuro, Buggy, Morgan
I added a new tier because they don't seem to fit within "strong" or "average". Don Krieg and Arlong may have been powerful, in East Blue, but not definitely not in the Grand Line. Ok, we haven't seen Sarquiss fight, but he is worth 38mil. Should be something. Kuro Buggy and Morgan are "possibly-s" because, while Kuro does have a neat ability, they aren't exactly Grand Line material.
average: The rest of the numbered agents, Don Krieg's lackeys, plus Gin. Arlong's lackeys, Alvida, Jango, Fullbody.
I think they're just average by Grandline standards. Gin should be above them, though. "Lackeys" here refer not to the ordinary pirate, but to those who are a little stronger. Though these are "average", they are still stronger than normal pirates in the "blues"
meh: Arlong's and Don Krieg's henchmen. the Billions of Baroque Works, normal marines.
I think normal Marines deserve to be ranked higher than ordinary pirates.
weak: normal pirate crew's henchmen.
I don't see any place else to put him. He'd be much higher up if he had a Golden Den Den Mushi though, but I'm assuming he doesn't even have Funkfreed. If he does, he'd be in "meh".
EDIT: Erm, Water is actually a bad conductor of electricity until you add impurites like salt in them, thus introducing ions which conduct. Ice is a bad conductor too.