Nuke is like a high powered shot from a pure INT.. or someone using a lot of MP to provide a blow that will severly crippled a foe... usually the damage ranges from 2,000-8,000 HP losses... theres multiple types of nukes in the 3 major imbues.. lightning, ice and fire.. if you watch some high lvl's duke it out , outside of jangan or donwhang you'll see them drop nukes on eachother occationally...
Imbue is the choice of skills to be using.. fire, ice, lighting, force.. those are your 4 imbues.. and of course only 3 of them do damage.. force is for healing..
A nuker is someone who drops nukes for high damage.. Usually its a person that is pure INT and cannot afford to take hits because of low HP.. so they drop nukes on ppl..
What is best? It's purely your choice.. You don't have to take anyone's word for what is best.. Fire is more destructive, but lightning can stun ppl and be very damaging itself... Personally, I like fire because of the burn and high damage ratings of it... I'm like 5:1 STR so I just like being able to brute force my enemy.. Umm, I do find however most pure INTs like lightning or ice for dropping their nukes... And a pure STR or hybrid STR will usually drop a fire nuke..
Pure INT or STR... This is gonna be argued till the end of SRO.. and forever beyond that... Most ppl will say pure INT will get owned by a pure STR.. but it really depends on distance and the pure INT's ability to put some crippling blows on the pure STR.. pure STR's have LOTS of HP.. by the time they hit 69 they can have 13,000+ HP.. and that can substain quite a few blows
INTs on the other hand will only have a fraction of that.. and will not be able to take that many hits from an STR... Pure INTs can and will lvl faster then pure STRs... but pure STRs are the tanks.. they are hunters and the ppl who are feared on trades.. the last thing a pure INT thief wants is a pure STR hunter a few lvls above him, lol... thats why most ppl opt to have hybrid nowdays.. hybrid can find a decent balance between the two.. or go a little more in one area then the other.. I, Myself.. Am 5:1 hybrid of STR.. I have the ability to drop nukes and take quite a few blows.. I have 3000 hp and 1700 mp.. so that's a decent balance IMO... my phys balance is 89, my magic balance is 46... But, I'm prolly gonna up my INT a bit to make my magic a bit more powerful.. I've heard of countless encounters were pure INTs have got their asses handed to them by hybrid spears... same with pure STRs getting crumbled by hybrid spears.. IMO a hybrid spear is the most deadly in the game.. they can't take the most hits, or put out the most deadly hits.. but they have a common balance between the two and can generally do very very well against mostly anyone...