Let's start with the First:
1. We know from the Wish that if the Master was released, the worse he can do is take over Sunnydale and open a human canning factory. So the Hellmouth opening is a bad thing, but it doesn't seem as bad as you'd think, again from the Wish. If nonthing else, the I team takes him out.

2. The Judge is assembled and dismembered. And all it took was a really small rocket.

3. Angelus wanted to •••• the world into Hell. Sounds like a Krang plot from TMNT, but this one could be really bad.

4. Sisterhood of Jhe opens the Hellmouth. Essentially this means more demons are roaming the Earth, and we can't have that.

5. Mayor turns into giant snake. Umm, this really doesn't rank as an Apocalypse per se. I mean, so he kill a few people, and does, I don't know what. And why didn't the mayor do anything about the Hellmouth opening parties?

6. The Three Suicide demons: Hellmouth Opens. The I-team takes them out.

7. Adam constructs a army of cyberdemonoids. Yeah, did you see the judge, again?

8. Glory opens the gateways. Yeah, this could be bad. It's an Apocalypse.

Sweet destroys Sunnydale and leaves with Dawn. Not quite an Apocalypse, but is there a bad reason to bring up "Once more with feeling?"

9. Willow burns Gaia. That could have been bad too.

10. Tradition dicates the first will try something that forces the Scoobies to act.

11. Okay, Angel has real Apocalypse going on. Excepy for the fact it's localized! So LA becomes a Demon City? It's not the end of the World.

So, there are three circumstances in which the end of the world could actually come about.

7. Adam constructs a army of cyberdemonoids. Yeah, did you see the judge, again?

I always wondered why no one ever did try to take a rocket to Adam. Seems like it would have ended that threat pretty quickly.