I've been wondering what sort of weapon I want to craft as an end game goal and the thing I keep hearing is lightning weapons are very powerful. I've got the lightning spear from Sen's Fortress at +4 (don't know if I want to waste a Slab on +5) and I must say I agree, it's pretty powerful.

I made a Divine Estoc +2 to kill skeletons and the numbers don't seem to add up. I know that divine weapons scale more off of Faith and less of of Str/Dex, but the base damage should have been at least comparable to the Drake Sword, assuming you add the physical and magic damage together. I was fighting some hollow knights in Undead Parish, do they count as undead? Does it only deal the extra divine damage to certain enemies? Do hollows count as undead?

What are the advantages/disadvantages of Magic, Enchanted, Occult, Raw, Fire, and Vanilla +15 weapons? Are there any Demon or Dragon weapons that, when upgraded, compare to other fully upgraded weapons?

Reading this only reminds me how far I have left to go. Damn you.

If you upgraded the spear to a +4 already, you might as well go all the way, unless you're into the idea of grinding (a lot). Fire and lightning don't scale with your stats, so on a first playthrough they will be very powerful. But after they are maxed you'll never do more damage, so you get diminishing returns. The other upgrades are about scaling with your stats (raw less so than regular) and/or splitting damage between different damage types.

The different ascension basically just dictate what type of damage weapons do and what stats they scale to.
 
Magic/Enchanted scale to Intelligence, Divine/Occult scale to faith. All other weapons either only scale to strength/dex or don't scale at all.
 
Magic weapons do more damage at +10 but Enchanted does comparable damage at +5, which is its max. The same is true for Fire compared to Chaos and Divine compared to Occult, although Occult is different in that it scales better to Faith than Divine weapons. Electric weapons max out at +5 and do comparable damage to +10 Fire weapons although they are a little weaker.
 
Magic/Enchanted weapons deal magic damage, Divine/Occult weapons deal holy damage, Electric weapons deal electrical damage, and Fire/Chaos weapons deal fire damage.
 
The other two types of asencsion are Raw and Normal. They compare to the others as well in that Normal is very strong at +15 but Raw weapons deal comparable damage at +5.
 
Basically if you want to create a good weapon quickly go with either Enchanted, Occult, Chaos, Electric, or Raw based on what you want. If you want to build an endgame super weapon build one of the other categories.
 
Each Demon and Dragon weapon has unique stats as all weapons do, so it is hard to compare unless you give me two specific weapons. There are certainly Demon and Dragon weapons that do incredible amounts of damage when fully upgraded, for instance Quelagg's Furysword is quite popular right now and the Dragon Greatsword has one of the highest damage stats in the game.

Note also that having multiple damage types can be a disadvantage, as all the enemy's respective resistances are used. For example:

Say the enemy has 100 phys res and 50 lightn res. Weapon A has 300 phys dmg, Weapon B has 150 phys and 150 lightn.

A: 300-100=200 dmg dealt

B:150-100+150-50=150 dmg dealt

For some weird reason i hate elemental weapons since they dont involve any planning and scaling skills. I dont know, i just hate every weapon that everybody uses maybe hehe. I cant tell you therefore on the scaling of other weapons for that matter since i dont use these elemental one trick ponies. My favourite Demon Weapons are so far the Lifehunt Scythe (+4) (damages you HARD if you dont be careful but has a killer damage) and the Chaos Blade (right now +4). The latter is fun in pvp since it adds the twist of having to connect 3 consecutive strikes to get bleeding in and the Damage ist really really good all in all.

To KIND OF answer your question: +15 Weapons are the ones i prefer over Boss Weapons, they are the hardest to get, but supposedly the strongest ones if handled right with the right stats. Just make sure you CAN handle the weapon (skillwise). Another point to note is that if you morph a Iaito for instance into a chaos blade it looks the same but gets a different moveset than prior so always check that if you upgrade into something else like boss soul weapons.

And rule of thumb... if you love your weapon NEVER upgrade it to Raw, you gain a quick boost to damage but it doesnt go down the road to effectiveness as going all the way down to +15 even close.