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
>