Alpine, Pioneer / Pioneer Premier, Eclipse, Denon, Sony Mobile ES, Nakamichi, Clarion etc. Generally brands that people like very much, yadda yadda yadda.
Then you've also got your JVC, Kenwood / Kenwood Excelon, Sony, Rockford Fosgate, Panasonic, Blaupunkt, etc. Some people would consider these some of the best, due to past experiences, a personal bias, or just factual evidence. So it really depends.
i like the pioneer or premier because to me you get more features and what not with every dollar you spend on it then say an alpine but alpine is a very good headunit. now that i bought my premier 750 i don't know if i will ever buy another deck again unless this one gets severely outdated or something like that.