So far, the plan I find most impressive is the genocide plan. The advantage of a lethal super-plague is that it fulfills the #1 goal of destroying the existing social order (and all faith in it) most effectively. With a properly seeded memetic environment, a monumental disaster like that would leave you pretty well situated to take over the world.
The problem with the plan is that it is beyond your magical ability if you can only cast 6th level spells. The existing rules offer no guidelines about how you would go about creating a new virulent disease. You certainly could create a new magical disease as the product of an Epic level spell, but such a spell would be way beyond a 'mere' 13th level wizard. You could of course make use of an existing magical plague - perhaps releasing into the world a crop of vampires, werewolves, ghouls, or whatever reproducing magical scourge you could come up with - but this plan has two problems.
First, an outbreak of vampires or werewolves is probably EXACTLY what the MiB organization is geared to fight. You would be playing right into thier strengths, and almost certainly you'd find yourself in the position of your typical fantasy villian who has had a party of high level adventurers (or several such parties) sic'ed on him. As everyone who has been in a successful PC party and played D&D, CoC, Chill, or what not knows, this is not a good position to be in. Secondly, even if your plague does manage to overwhelm the world's defences (and remember, eventually the world militaries with all thier high tech fire power would adapt and be brought into play), you've still got a problem in that the plague has probably advanced far beyond your ability to control it. You may well have brough down society, but that isn't the same as having control of the world. Instead, the world would almost certainly be under the defacto control of vampires, werewolves, ghouls, or whatever.
Now, creating a plague capable of destroying society is NOT beyond the technical means of current world governments. For example, the Soviets did manage to create at least one super-plague that I'm aware of that could have quite potentially wiped out the entire human race had it been released. So if a magician (or anyone else) wanted to learn enough gene splicing techniques, and had enough money to create a lab that was sophisticated enough that he wouldn't kill himself playing with one of his less sophisticated creations (drug resistant antrax, or a delayed onset small pox for example), then he could wipe out the world with relative ease.