I used to work for Pioneer (11 years!!!) so naturally i went for a Pioneer DEH-8400BT http://www.pioneer.eu/images/product...00_gallery.jpg
Whatever brand/model you choose, I would give you these advises:
1) Get one with bluetooth handsfree, and DO connect the mic and pair with your smartphone. The feature is not that expensive, and you relief yourself from a significant crash-risk and risk of being pulled over by the police for speaking in phone while driving.
2) Choose one with the USB-plug on the backside, and lay the USB wire end in the glovebox. Much easier demounting of front panel, no more risk of 'chopping' off a frontmounted USB stick, and still all your easy music available.
3) Be aware that the sound quality depends on the speakers, not the radio. Even though I feel tempted to say that Pioneers stereos sounds better than the rest, the differences between the different brands is really small. The loudspeakers however has a HUGE impact on the sound quality, so don't skimp on these: Buy the best you can afford and manage with maybe a smaller model of stereo.
One speciel feature of the above mentioned Pioneer model is, that you can program it to have the rear-speakers act as a subwoofer ( obviously only interesting for GTV owners). So the deepest bass is routed only to the rear, and that has two benefits:
First of all the 'soundstage' will come only from the front, which is much more natural, and how you would listen to your stereo at home.
Secondly the front speakers are releaved from trying to play bass. As they are allready very small, attempting to play both midrange and bass is no easy task, and by asking them only to do the midrange the sound gets much much....MUCH cleaner, and they can even play louder this way.
So the rear-speakers may not be genuine subwoofer capable, but just by splitting up the sound you get quite an improvement in the sound, and that is without spending precious bootspace for amplifiers and subwoofers. Yes, in the backseat there will be mostly bass, but you probably never have anyone sitting in the backseats anyway.
I would happily answer questions about sound or installation.