My vote is for Angelus.
The Master:
Not a terrible plan but he was hindered by being relatively easy to beat. Among the BTVS big bads, he's probably got the best plan after Angelus.
Angelus:
I think Angelus probably had the best plan of the lot, and came the closest to winning. When you really sit back and think about it, the only reason he lost is because he was a little too obsessed with Buffy. If he'd just killed on the numerous occasions he could have (seriously, he could, and did, enter her room at will while she was sleeping), he would have easily won.
The Mayor:
Not a great plan. It might have worked in about 1500, but in the 90's, The Mayor wouldn't have lasted all that long, with or without Buffy to oppose him. Even if he kills Buffy and successfully ascends, what comes next? The U.S. military obliterates him.
Adam:
I don't even really know what the hell Adam was really after, but even if he'd defeated Buffy and the gang, he wouldn't have gotten all that far. At the end of the day, he was just one person. I mean, he wanted to...what? Turn the whole world into half-robots? Doesn't scream genius.
Glory:
Glory came mighty close to ending the world, but she could have had she not suffered from super-villain plot stupidity. I mean, after it becomes clear that Buffy is a problem and she knows Dawn is The Key, why not just kill her? She easily could have. But she did come close. However, her entire plan was contingent upon Buffy or Dawn not sacrificing themselves, which was obviously a pretty big hole.
Dark Willow:
I don't think Dark Willow was stable enough to continue on for much longer. She certainly could have wrecked a ton of havoc and damage upon the world, but she was clearly under so much strain that she couldn't really handle it for much longer.
The First:
After maybe Adam, the worst plan of the bunch. S7 tries SO hard to build The First up as this unstoppable menace the likes of which Buffy has never faced before but...it was really kind of dumb. The First, even without Buffy in the mix, had no real game plan. I mean, it wanted the Turok-Han to overrun the planet or something. Well, for starters, the Turok-Han are a mindless rabble without any tactical ability who can only operate at night. That's...not a good start. Even if The First kills Buffy and emerges with its...we'll call it an army...it wouldn't have lasted all that long. Again, the U.S. military, hell just the U.S. Air Force, would have annihilated the Turok-Han. I mean seriously now, let's think about it. Sunnydale was already evacuated. The worst that would have happened is that the world finds out about vampires. Considering the might waiting in reserve, Buffy really wasn't that important to S7.
Wolfram & Hart:
Beyond Angelus, who came literally close to ending the world, Wolfram & Hart is the only big bad who is actually winning (or at least, they were prior to the comics). All of the BTVS big bads tend to go for the big, climactic knock-out blow on humanity. W&H does not.
Jasmine:
She's up there too. Her plan was actually working for a time.
