There are MANY good head units that will work well - as in hundreds.
The "standards" in the industry would be Alpine and Pioneer. Kenwood, Clarion, JVC and a few others also offer good quality.
In general - features, quality etc are going to go hand in hand with the price tag. For $100 you can get a basic good quality unit. For $150 a pretty nice name-brand with most of the music specific options covered and getting into aux input capability, USB inputs, some basic crossover and equalizers etc. At $200 or so you should be able to get everything you need and start loading up with options like high voltage preamp outputs, HD radio, ipod control, etc. Beyond that you start getting into complex equalization, customizable graphics, DVD capability, video displays and on and on.
Some possibilities -
(I'm assuming the car has a standard 1 din opening)
http://www.onlinecarstereo.com/CarAudio/p_25211_Pioneer_DEH-P5200HD.aspx
http://www.onlinecarstereo.com/CarAudio/p_28579_Kenwood_KDC-BT648U.aspx
http://www.onlinecarstereo.com/CarAudio/p_28425_Clarion_CZ401.aspx
http://www.onlinecarstereo.com/CarAudio/p_29337_Alpine_CDE-123.aspx
There are a lot of other good ones.
Just don't get sucked in by the off-brands that seem to be loaded with features vs the price. Having a lot of features is great, but not if buttons stop working and lights go out after just a few months, or weeks. If something seems unusually inexpensive there is usually a logical explanation for it.