I use Waves Renaissance compressor the most.
i also use Waves L1 on tracks and busses, but usually only just kissing the meter,
sometimes i'll bury the meter on bass tracks, depends on the dynamics of the song.
i also like Waves C1, i use that across the master buss and mix into it...
i use compressors wherever i want to control level.
across drum busses, guitar busses, individual vocal tracks, bass tracks, percussion.....
you really don't know where you need them,
until you have an active mix going.
if i could go 'faders up' on everything, and have it all sit pretty,
i'd never use compressors!
actually, i can do that now,
but it's not the sound i want.