All of it.
If I don't feel like one job today, then there is always a day when I will.  
After all, I do it to have fun.  
Pretty much my philosophy, too. 
Right from the point when I start a model, I have a mental picture of what it will (should) look like when finished, and this keeps me going when I strike a difficult area. In particularly troublesome bits it's sometimes best to leave it for a while - days even - then come back with a clear head (as long as you haven't been partaking of some medicinal alcohol 
 
 
 ) and start again. Having done this, it's remarkable how often a seemingly impossible job becomes easy.
However, if pressed as to my most enjoyable job, I would say fitting the details which really bring a model to life. 
Peter.