Best suspension brand?
Honestly, it really depends on what you want to set it up for. Comfort/Street Handling, Drag Racing, Road Racing, Autocross...all require slightly different setups. Brand is more of an opinion, so I'd actually just check some forums. Google search "Mustang Forum" and see what forums are good, use the search buttom on the forums and see what other people have tried, what they liked and didn't like. That is honestly the best way, and the answers will be 100% better than anything you'll get here. Good luck and happy searching!