My most used is the weather widget...it's usually right and I'm wrong.
My favorite instance of this has been going out of the house in flip flops because I thought it was wrong...on my way back to the car from my outing I got to walk through an inch of water and get soaked in the process...
My favorite widget however is the flight tracker, which I've had the pleasure of using once to pick up my pops from the airport. I saw the plane was 30 minutes early so I left the house earlier than he told me. I got to pick him up earlier than he thought. Good times.
Least favorite...iTunes widget. Soooo laggy and buggy. Looks neat I guess though.