Angel vs The Black Thorn definitely tops the list. He forced the Senior Partners to send all of Los Angeles and its entire population to hell sooner in order to stop the Senior Partners from sending all of Los Angeles and its entire population to hell later.
Buffy activating every Potential in the world is only slightly more reckless. She turned several thousand girls into superhuman killing machines without preparing them in any way for what was happening or why. The "potential" for disaster is incalculable.
Adam's plan to turn everyone into half human/half demon hybrids served no purpose whatsoever.
Angel's partnership with Twilight. If Twilight had the power to transfer the entire population of the planet into a new universe in the blink of an eye, why didn't it just do it? What was the point of fighting Buffy, her Slayer army, and all of her allies first?
And let's not forget Buffy's plan to defeat the First evil by taking a couple of dozen inexperienced teenaged girls into the pits of hell armed with little more than sharp pointed sticks with the intention of defeating an army of ubervamps whit no assurance whatsoever that Willow would be able to pull off her Potential activation spell, which in and of itself was totally irresponsible. Then, making matters even worse, leaving six normal humans above ground to prevent a second army of Bringers from attacking her from the rear.
