Is astar the most appropriate for pathfinding

A* is just one of many algorithms and methods than can achieve pathfinding. It depends on what you're really trying to achieve. Do you care about processing speed? How well it works? etc.

It depends on how busy your levels are. If you are making a flight simulator, avoiding other aircraft can easily be done with steering behaviors. If your level looks more like a maze, you'll probably need a graph search algorithm like A*.

I can draw you all sorts of situations where flow fields break down. The most common one being a "C-shaped" obstacle of some sort. How do you devise a flow field that allows for reaching goals both in the "C" and on the back side of the "C"? You are either going to have a flow that allows movement to the interior of the "C" or you are going to force things to flow around it as if it was an "O" instead.

It really depends on what you're really trying to achieve.
Avoiding other aircraft can easily be done with steering behaviors.