I'd say Nora Chandler.

She was even nice when she got caught :)

I have to admit I adored Faye Dunaway in that episode and would had to have let her go so shows that I wouldn't be a great cop!

The "Nicest" villain may be hard to choose,but the "Worst" ones got to be from this episodes:
1.Lovely but lethal
2.Lady in waiting
3.Most Crucial Game
4.Publish or Perish