That is a very deep question. For me, it really depends on the quilt. One of mine "Forest Fire" was easy to pick a quilting design, it was a quilt for my nephew who is a hot shot for the forest service in NM. I did a green diamond quilt in scrappy form and quilted it with red/yellow/orange thread in a flame pantograph
was perfect. Another quilt "Portals to the Orient" stumped me for ages. I finally asked a fellow Long Armer on here for advice and she pointed me in a direction for ideas, they were perfect. Still need to get that one done, but have the quilting down
just need the time.
Generally though, the quilt style/color will tell me what would be best with it. Other times, I have to work at it to figure something out. If the quilt speaks to me, I can "see" it finished in my head and know exactly what I want and where. If it doesnt speak. I do some drawings on my dry erase board to get an idea. If I actually have the quilt loaded, I will take a piece of plexiglass and draw out what I want and lay it over the block to see if it does what I want it to do.