I'll go first.
My go-to knot is the improved clinch knot. Hands down my favorite knot for just about everything.
I very rarely use braid, but when I do, the Palomar knot is my go-to for braid.
As for joining two lines, I didn't currently do that, so I have no preference here, although I do remember tying what might have been a uni knot at some point in the distance past. Or maybe it was a blood knot?