Sephiroth both rounds.
Sephiroth should win round 1, definitely wins round 2.
I've doubted some of the calcs before (I do not buy Hashirama having massively hypersonic reactions), but I know that several members of SOLDIER are bullet-timers. Since Sephiroth made the other members of SOLDIER look like trash, it reasons that his speeds are firmly in the hypersonic range. Until I see more evidence I wouldn't say that he'd definitely beat Mihawk in round 1.
Ah I remember the KH1 Sephy fight. I'd just finished the game and was bored so I went to the coliseum. Then I saw these two new cups, Platinum and Diamond. This looks cool I thought so I went straight into platinum and then all of a sudden the music starts playing and I was like who the hell is this cool guy (having not played FF7). Needless to say he kicked my ass.
However I leveled up and I eventually beat him, then I just kept beating him to level up to 100 because he dropped a crap load of experience.
Also Sephiroth probably wins this, but I'm not so sure as most of the people seem to be here. Mihawk's not got many feats but he was easily able to deflect a cannon ball and could react to Gear 2 Luffy easily. Plus he's more destructive. I think it would be a good fight.
@Fehafare: They show all these calcs which are purely fan based but then you watch FF7 Advent Children (Which is where everyone is at their most powerful) and I didn't see anything that was remotely close to these 'Mach 100' calculations that I see.
I just don't believe fan made calculations at all, they're always ridiculous. If they can move at Mach 100 why do they even need airships or motorbikes? Why aren't all the battles over in a second? I think the movie is a far better resource to use for feats than the game mechanics. Or in game cutscenes like the ones in Crisis Core, which are clear and much easier to quantify.
That's just my opinion on this.
@ohgodwhy: Pretty sure the calcs are based upon Advent Children and not the original game.
And they still use vehicles because they can't actually cross large distances at those speeds, those are small bursts for a few meters at best, only usable in combat.
The battles aren't over in seconds because no one would bother to watch a movie like that.
Calcs are pretty much the best way to determine the strength of feats if they are not self evident. I largely dislike the mentality off "lol i don't care about calcs or numbers, i just go by how flashy the scene is".