Some equipment I've got or have had in the past.
NICE HU
higher end Sony's (Doesn't skip with tons of bass, decent quality)
Any Alpine (Great quality skips when CV Strokers start to kick hard)
Nice Sub Amps
MMATTs (hellous power, more noise)
Positives-type D amplifier, never gets much above 25 degrees C.
Nice Mid Amps
U.S. Amps (Great quality output, could cook eggs on it)
NICE Subs
MMATTs Juggernauts (Nice bass, capable of nose bleed productions)
Cerwin-Vega Strokers (Nice bass, capable of nose bleed productions)
Older Precision Powers before wussification (Hit hard without pain)
Earthquakes (Nice but can't hang with the 3 above)
Nice mids
Older Precision Powers before wussification
Alpine
Tweets
Again older Precision Powers before wussification
Qualities to check for in a HU
OUTPUT power and number of outputs
Signal to noise ratio
Power Consumption at peak output
Remote controller
I personally like my 6-disc in dash standard din size HU
Color of HU, does it come close to matching your dash. So you won't have an eyesore.
Does it have lots of nice games built in like many of the Clarion HUs have when you are in need to pass time while your significant other shops in the mall for hours.
Check for support of auxillary audio inputs for those of us who like having X-boxes in their car with good audio.
Does it even have amp outputs? I have seen Sony's without it.
Removeable faceplates are nice.
There used to be a company that I dealt with a long time ago called Carmedia1. They are a good bunch with great prices. I highly recommend doing business with them when you make a decision.