Why fifth?
Well, honestly it was my first, and I actually think it was the best of the editions as far as completion's sake is concerned. I love the cards with the Wayne England symbols and the vibrancy of the edition as a whole.
We also played a hell of a lot of whfrp back in the day, (Shadows over Bogenhafen) and even though I might have played my majority of games under sixth and most of seventh, I still feel that fifth was the truest representation of the universe for me. It wasn't the stripped back army books of sixth, or the bland attempts to fix up the content of seventh and eighth.
With all the Characters that were forgotten as we progressed coming back to life for me now, like Skaw the Falconer, Roland le chasseur, Armand d'aquitaine. But most importantly Gotrek and Felix.
The only thing that is really missing were ogre kingdoms as a stand alone army, but honestly I like that they werent a full blown race as the weren't in the "hub" for gaming civilistation and were basically the mercenary race their later background tried to reflect, but I think failed to replicate on the table in seventh and eighth.
And who can forget Shadow of the Horned Rat, my first RTS game...