Best when you just start it.
You know, when you don't know exactly where are you going or what kind of synth you're gonna use, how the melodies will be, it's really exciting!
When you create music out of nowhere.
Worst...well the final mastering
Also when you have almost finish your track, but you must add some more elements, but you're afraid you're gonna ruin it.
Also when after hours and hours of work you just realise that the track sucks and you completelly delete it.
Yours?