This is not a 'best of' or any other similar kinda list. It's just a random list of various plugins I have used at various points of time and liked. You may want to check these out ..
DFX Audio Enhancer
Audio Enhancement for Winamp. Renew stereo depth, boost your audio levels and produce deep, rich bass sounds with DFX.
Tape Restore Live
A free Winamp plugin that improves the sound of cassette tapes.
Audioburst FX System
It enhances digital audio by increasing the power output from your speakers.
One of the best free public-domain reverb Plugins
PaceMaker is a Winamp DSP plug-in allowing you to adjust tempo, pitch and speed independently. With just a couple of more tweaks, and a bit more precission it could be used by DJs for mixing MP3s.
DSP for enhancing audio. Analog modeled EQ, tube compressors and amplifiers, room simulators and more.
Qmp3D from QSound Labs is a software plug-in for maximizing your audio listening
Shibatch equalizer plugin
accurate equalizer plugin for winamp
Stereo and mono sounds are extended to surround sound