The best "cat-back" exhaust
 

There seems to be many different kinds of exhaust brands but what im looking for is an increase in horse power and a nice deep sounding cat-back exhaust.

Ive seen many people on boards with Mac, Flowmaster, BBK, Dynomax..anyone have an opinion with which one is the best for the buck. Ive seen some flowmasters go for $300 cat-back. Any help would be appreciated.

Cat back systems are good for sound, that's about it. If you want to see a performance increase, you would want to pair it up with a good X/H-pipe.
As far as sound goes, my favorites are Dynomax and Borla, and Borla is just plain expensive...

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1998 Mustang GT
14.18@100.56
"Traction Impaired" or "Nitto Needy"

BORLA works for me....Gives great performance and sound like no other. They also last forever..........

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I have the DynoMax Super Turbo catback system and found it to be good quality and a nice, mellow sound; but the 'sound issue' is totally subjective and also dependent on the H-pipe configuration and choice (or absence of) headers.

No cats/headers will make any cat-back system louder and factory cats & headers will make it quieter (and more restrictive).

Borla is a very high quality manufacturer but you pay for that quality, of course.
Flow Master is generally loud but decent quality but as a previous post mentioned, you won't see more than a few HP gain with just mufflers and pipes - you need the headers and H-pipes replaced to get a real power increase, simply because the factory mufflers/pipes are not that restrictive but the (4-cat) H-pipe is, as are the headers, although they don't rob as much from a stock engine as they would on a modified Mustang.

To answer your question: I recommend Walker DynoMax as the best for the buck.