Uh, a super volcano erupting would be a nice way to go!

EDIT: Though I don't suppose it would count as an armageddon

There wont be any apocalypse we cant over come.

Some will always survive.

That's not quite true, at least for natural disasters.  There are plenty that we currently do not have the technology level to guarantee survival (luckily these are the ones that tend to have astronomical chances of actually happening).  I do agree though that being adept with technology makes us far more resiliant than any other species on the planet to being rendered completely extinct.

Any astroid/comet we can take out.

Any change on the earth it self will still leave some areas with peopel friendly enviroments. 

Only the sun can really take us out, ands that pretty much takes out the whole palnet/solar system. 

Ice caps have melted and grown before humans weren't around, but man is able to change his environment and is capable of doing such things.  And humans aren't the first living thing to have that claim, I'm thinking of ancient bacteria and photosynthesis.  Besides, mosquitos with three foot wingspans that were around in previous hot periods doesn't sound too appealing to me.

I dont see why you mentioned your second point.  What matters is how much overall effect something has.  Methane is obviosly another greenhouse gas that has more of an effect per unit mass but there a lot less of it.  Its the overall effect that we are concerned with, and yes methane is part of that equation.

Again, I dont know how this pertains to the argument.  We put CO2 into the air and the change leads to an increase in overall temperature, and that current increase is in terms of one or two degrees C, which isn't a lot in the grand scheme of things but can have major consequences as far as we are concerned.  If we put more CO2 into the atmosphere than natures puts there we'd start to resemble Venus.  The natural level of greenhouse gases results in our current climate, and I prefer to stay closer to our current norm.  I dont want to put entire ecosystems on the brink and cause mass human migrations from global warming.  We are going to run out of hydrocarbons soon anyway, so the quicker we try to switch to solar and/or nuclear forms of energy the better off we are.

Nuclear War, One country will miss fire weapons through false intelligence and or electronics malfunction, which will cause a retaliation and so on.

Runner Ups:

• 
  Bi-Terrorism
• Asteroid
• Burning of the Earth
  • as "ManusJustus" said, the Sun Expanding at the end of it's life cycle, and burning the Earth, eradicating humans from existence is the way that we shall go if none of the above succeed in doing so... 

There are my four ways that I see the "Apocalypse Scenario".

Whether it is theoretically possible that man could cause climate change, when you crunch the numbers it becomes clear that it is highly unlikely that man is causing dangerous climate change ...

While the IPCC predicts a temperature sensitivity to a doubling of CO2 at 3.6 degrees (Celsius) it has been observed at (roughly) 1.0 degree (Celsius). What this means is that humans’ production of CO2 has accounted for an increase in world temperature of (roughly) 0.6 degrees, and in order to stay below the 2 degree limit the IPCC has set we have to limit our CO2 output to (roughly) 6 times our current level.

     Is that that asteroid that is suppose to possibly impact Earth around 2035...Because I remember hearing something on the Television about that?

     I was just too lazy to look it up myself and thought that it would be easier to ask you quick...

too many people. the earth will literally start to spin off its axis when there are too many people weighing it down.

or something like that.

An alien Invasion. THey will enslave us and terminate us after they harvest all of our resources.