I use the Waves Masters Bundle. But I don't call what I do mastering, Just processing. Waves is the best I've found and I've tried Ozone, Dave Brown's plugs, and a number of others on my way up to the $600 or so I reluctantly coughed up for the Waves set.
But it's a night and day difference. When I think of the things I routinely spend 600 bucks on (tires, clothes, TV's, lawn mower, travel) , the Waves bundle was a best value - after
my Beyer M160 ribbon mic anyway. Or maybe before it. Yeah,
before the mic.
In freeware plugs, try the slim slow slider multiband compressor, Paris EQ, and Blockfish limiter by digital fishphones. All VST stuff. They get a better sound than Ozone IMHO, but nowhere near the Waves bundle.
Good luck and keep experimenting,
Tim