Which ones?

Try e.g. Audacious with its "plug-in effects" such as BS2B and Crystalizer.

Note that these basically add *distortions*, and it's highly personal as to which effects sound like an improvement

LADSPA and it's spinoff LV2 are the best plugins when variety and solid development are considered. They are de rigeur in serious DAW work as with Jack, Aqualung (and other players) and the mighty Ardour, a truly professional DAW suite. The main dev of Ardour also developed Jack and contributed to LADSPA. LV2 will likely supplant LADSPA before very long but having grown out of it, and the reason for the spinoff being compatibility, that's a good thing. Incidentally, if you prefer the more limited but friendlier Audacity note LV2 that Audacity already supports LV2.