I have several favorites of those I currently own. They include the Gayle Bradley, PM2, Dragonfly, Manix 2 LW and Stretch. I'm also quite fond of the Cat G10, Chicago G10, Sage 1, Military and Delica 4. The model I've owned the longest is the original Endura and it's an individual favorite due to that long association.
If I absolutely must pick a single model, I would have to go with the Dragonfly G10 since it's the one I carry the most.
I don't have a favorite of those I don't own.
I've only lost one Spyderco--an original Delica. I guess I regret that although I think the current Delica 4 is a better knife all around.
I've never sold a knife and I don't dream of knives.