Under OS 9, My 2 absolute favorites are TC Master X3 (PowerCore) and 1176 (UAD 1 Card)... but there are 2 issues with Master X3...
1) you need an original TC PowerCore PCI Card (NOT element) to run the plugin
2) Under OS 9, the Master X3 Plugin was custom generated for each card by serial number...TC (even before they abandoned Powercore), was giving me a real hassle about making Making the Master X3 plugins for the cards I bought 2nd hand... In 2010, I threatened them legally, and explained how the box clearly states that the Master X3 plugin is included and to contact them to get it... They finally found their OS 9 Unit and generated 2 more Master X3s for the Cards I bought; don't know if they will even answer the phone these days about PowerCore
Still a worthwhile investment for the other plugins included
The UAD 1 "Mackie" version (get one on eBay for less than $80) is amazing, the Limiters, Compressors, and Eqs surpass almost all native OS 9 plugins and easily give you pro mastering results