Depends on what you're trying to do. If it's drag, control arms is a good place to start that's affordable. If it's road race, springs are affordable, but then you might need to follow it up with panhard rod, camber plates, bumpsteer, ball joints, new alignment etc to correct a few things.
It's a little pricy, but Tokico D-spec adjustable struts and shocks was by far the best suspension mod I made.