My favorite, and the most fun, one that I will remember for a long time was:
http://www.geocaching.com/seek/cache_details.aspx?wp=gcjq2x
Beat the Heat Event Cache in Prescott  (GCJQ2X)

The most extreme, for me at 62 + years of age and bad knees, was:
http://www.geocaching.com/seek/cache_details.aspx?wp=gc2127
Dynamite Joe's Frijoles  (GC2127)
Here I did not believe the
Terrain  rating and thought there was a mistake, until the last 100 or so feet.
Boy was that hard, for me at least. 

Some of the cachers I met at the Event cache have powerhouse legs and good knees so you are not effected by large terrain numbers.
Under Extreme, I read about a cache that is under water and requires SCUBA certification, which I do not have and don't think I will try at my age.

There are a couple of others that come to mind but these come in first right now.

Bob Smith, Team Petite Elite


Gale wrote:
Ok, I've a question for the group.
 
What is your favorite, most unusual, most extreme cache that you have visited or heard about? Please post link if you can.
 
One odd one I remember, that got pulled, was a cache pinned at the bottom of an iceberg from a glacier in Alaska. No one had found it, although one team made an attempt at it, before it was archived as too dangerous.


Tsegi Mike and Desert Viking