My most memorable trade happened recently when I had put a Tepig up for trade in Black2. I was trying to fill my Pokédex, and was asking for a Crawdaunt, any level, any gender, I didn't care. 'Cause I was going to breed it with a Ditto anyway to receive a Corphish, or so I thought.
One day after my request, a trainer had traded me a Crawdaunt just as I wanted. But the Crawdaunt was level 100, had perfect IVs, a great nature and total killer moves...
I did not use that one to breed.
As for least memorable? I haven't really done any bad trades at the moment.