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This one is going to cause some arguement.
For Sephiroth I give him all his abilities, same with Alucard.
Personally I think this one goes to Sephiroth without a doubt. Him with all his abilities and out for the kill, there's not much anybody can do.
And if you don't know who Alucard is, he's from Hellsing, go do some research!

Wha....are you NEW!?!?!?

Sephiroth takes this one and quickly. Video game sephiroth would have won this fight easily, but now that we have the advent children movie to draw from as well, it's easy to see that sephiroth's speed, strength, and abilities are easily superior to those of Alucard.
Alucard really likes to screw around though, and I think the fight would go early to sephiroth. Now if Alucard took on his demon form it might be a little different, but we don't even have the slightest clue as to what he's capable of in that form, but it's certainly nothing a materia powered Sephiroth couldn't handle.

Have you even read/watched hellsing. Sephiroth has NOTHING that Alucard couldn't come back from, but just to save time, let me shoot down some of the fanboys favorite go-to posts:
"But he can summon teh meteo!!!1111"
-The meteor takes a friggin week to actually hit, and that's only if he has the black materia, plenty of time for Alucard to school him.

"He can blow up teh solar system!!!"
-Supernova is an ingame move that to this day I have never actually seen KILL any of my party members while in their 50s. In addition, it also can't do what it appears to, because he casts it multiple times. How can you blow up Mercury when you destroyed it 5 minutes ago already?!

"His advent children form is to fast and to strong!"
-He wasn't that fast to me, as I could friggin see his movements with my naked eye, and I need glasses, also, cleaving a few falling chunks of building in half is meaningless in the whole of shonen anime when you compare it to the likes of OnePiece. Regardless, it seems that Safer Sephiroth is his strongest form, because atleast then Cloud needed two other people to fight him, In AC, Sephy got owned by Cloud alone, so I'm not impressed.

"He has materia"
-If you take him at his strongest, he has no materia to use, because Seph at his strongest was Safer Sephiroth, and he had no access to materia, if you want him to have materia, you have to use the playable Sephiroth from the game, which would make him even weaker compared to Alucard.

There, in fact their is a big gaping flaw in even attempting to use in-game Sephiroth, as he is restricted by battle mechanics(as are materia). You can't give him one without the other. If Sephiroth can use materia, then he also has to wait till his turn comes up to actually fight, while Alucard has no such restriction. Either stick with AC Sephiroth as your fighter or don't bother.

Alucard's highest speed feat is bullet dodging, right? I haven't seen anything higher, and definitely not far higher. Sephiroth was on toe with Cloud who deflected and dodged huged barrages of bullets, not to mention he can deliver 13 slashes in a second with KH2's Flash attack. Endurance is the only thing he loses in really. Which is why he would lose, of course, but I just had to point this out.

Due to debate with Trevor I'll stick with KH2 Sephiroth alone. Who STILL has shown more speed and probably as much strength. Speed feats would be his normal combos and slashes that are super fast and then the Flash attack. 13 slashes delivered in a second. That's easily bullet speed. Although now that you mention it, when did Alucard dodge bullets? I can't remember so an episode number or something would be nice, usually he just lets himself get splattered, or uses illusory techniques to make it seem like he dodges, but he's never there in the first place.

As for strength. At that point in the game Sora was able to deflect building chunks (Genie Jafar in Agrabah second time through) and his arm strength was enough to hold back Cereberus. Then he'd fought Hercules. Sephiroth not only matches Sora's strength, but surpasses it when he breaks through the kid's guard. Alucard is tough, but the highest strength feat he has is being able to tear people apart 'like wet tissue paper'.

We already know from Nomura's interview that the FF characters DO fit in their timeline and KH's. The Ultimania Interview asked:

But after the fact the AC characters were there. And then:

Tifa from FF7. AC is apart of 7's story line anyway. Not to mention the Gamepro magazine interview.

If it's the same character then why wouldn't he have the same abilities? Although the building chopping and fast movement weren't even abilities that you can lose; it's feats.

EDIT: Canonicity is far to opinion based for me to convince you specifically they're canon to KH; I'm just trying to underline Sephiroth's feats.