I know it's hard to choose JUST ONE but you have to choose your favorite!
 
I'll go first!  
 
Even since I've been collecting, classic yellow rubber duckies have been my favorite because there are so many different molds and designs you can find everyday.  One particular duckie (or should I say set) that stands out from my collection is this one.  I actually got this through a trade on Duckplanet!! I love how it is so simple and the beak is slightly tinted.  I also love how the baby can be fitted right on top of the mama I haven't seen too many other sets like this one but it is one of my favorites.  
 

If I could keep just one duck, this is the one I would keep -
 

Havekidsdonttravel:  Could you please tell me where one could purchase such a duck?  I would love to add it to my collection.

My favorites are the ones that have the most sentimental value. I love my Cheese Rubba duck. That duck has traveled with me everywhere. Also this little duck with a red ribbon around it's neck that I've named Vegas Baby. It was one of the first in my collection. It's a really common duck....but it has a lot of great memories tied to it.