RNS510 if you want full OEM integration, however the nav is lacking and you can't have audible speed camera warnings!!
I had a Zenec NC2010 also which offers very close to OEM integration, has Igo which takes TomTom mapping with good sound warnings. Integrated Parrot bluetooth and so on
Kenwood do a very nice mk5 based HU, and there are others, it all depends on your aims & sound quality requirements.