Keyboard Assist Button not working?

For me it depends on what I'm doing on the specific track. In general I'll start with KK if it's a library that's fully integrated with my MK3. If it's a 3rd party library that isn't, I'll typically start with Kontakt. In some cases I use Soundpaint as the interface because it's easier to adjust various layers. There are even cases in which I'll switch. I might start off using Kontakt and after laying down the track I'll switch the track to Komplete Kontrol or vice versa.