Sorry, I need to nominate three..!
My musical weapon of choice is electric guitar and no plugin I've tried can match Scuffham S Gear:
Vocals are the most important element of my songs and the Waves Scheps 73 makes a massive difference to me. I've used it on every track I've recorded for many years, and it adds... I dunno, definition, clarity, helps it sit well in the mix - it's just magic! My settings normally look something like this:
Finally, a great way to easily enhance your drums - works great on snares, kick and overheads. Dead easy to use - Waves Scheps Parallel Particles: