I am a professional sound engineer and this is a somewhat generic question. The equalizer should be matched to the job is is required to do. Generally Klark Technic Ashley and DBX are higher quality brands.
I use Boss pedal equalizers. They are reliable, durable, and a great quality product.
Many brands of car graphic equalizers are offered at major chain stores at competitive rates. You have to compare the products and choose the one that appeals to you the most. Pyramid, Clarion, Boss, Kenwood, Pyle are among the best and most popular brands of car graphic equalizers You may find this link of immense use to make a comparison-http://www.sortprice.com/search-CQ-Car_Equalizers-Graphic_Equalizer.
Some major brands are Boss, dbx, Berhinger, art and DOD.
Equalizers can be both. Most likely it is a piece of equipment however the equipment may be digital and software driven. Equalizers are used to even out inconsistencies in other equipment like microphones speakers and even room dynamics.
Legacy have some good brands. Check out others here: http://www.bizrate.com/car-equalizers/
Starting at around $200, the Samson digital graphic equlizers are the highest rated. Pioneer is also an excellent brand!
Boss Audio makes great equalizers that are safe to use on aftermarkets.
Professional singers use equalizers compressors expanders aural exciters and various other enhancements. You would hardly ever find a professional model who hasn't had pictures touched up and likewise a professional singer without enhancements to their voice.
The best mic is the Neumann U87. It sells for about $3000.00.
In my opinion, Boss equalizers are the most consumer friendly products.
I like the dbx equalizers because the location of the connections is very convienient for installation.
Yes, all stereo head units come with equalizers built in.