Which Adware and Spyware are the Most Insidious?

the_dreadnought asks: "I was just asked today which adware and spyware are the most insidious by an acquaintance. He asked me if this stuff was really legal, or was it just not important enough for law enforcement to deal with? I know the porn stuff (not from experience,,,ok, from experience) that dials out to foreign countries is one of the more extreme examples, and Gator is well known, but if Slashdot readers could describe what adware and spyware they think is the sneakiest I would appreciate it. Also, any thoughts on whether some of this stuff is even legal, as it is almost certainly not ethical."