Depends on how much you want to spend and what features do you want.
Bare minimum is to upgrade the Canbus Gateway otherwise you will experience Battery Drain with these OEM units. You can get the 7N0 907 530 canbus gateways for about £20 uncoded or about £30-40 precoded.
Most people tend to upgrade to the European RCD510 or RNS510 stereos but it will need the additonal Bluetooth module should you want BT stream and have telephony. It also does not include any USB/AUX options by default, you will need to purchase additional wiring through where the ESP buttons are. And to make things more complicated not all RCD510/RNS510 have DAB. You can tell by looking at the back of the unit, if it has a black port then the unit has DAB. All in all the RCD510/RNS510 while one of the most aesthetically pleasing is also one of the most expensive OEM options you can buy - you'll be looking around £500+ for all the options fitted in.
The next option is the RNS310/RNS315, similar to the RNS510 but has a smaller screen. But the real ace in its sleeve is that it has Bluetooth as standard so you can do telephone (with the optional mic) and audio streaming out of the box, it also has an AUX port on the front so no need to purchase any additional wiring too. But again be sure to check that is has DAB as not all RNS310/RNS315 units have this. These units can cost around £200-350 ish.
If you're not bothered about DAB or Sat Nav then you have a few more options open to you. If you're looking for an OEM unit with lots of connectivity at the lowest possible price then you can't go wrong with the RCN210. Lots of features built in for the money, Bluetooth + Mic Built-In (Telephone and A2DP Stream), CD Player, AUX, SD Card, USB and iPod Control out of the box, these units can be had and fitted for no more than £50 (I had this for a few months and its been superb). The only minor complication is that it uses the ISO interface rather than the quad lock so it needs an adaptor and additional CAN wiring from the Air Con unit to get it all working.
If you want something a bit more upmarket and have a touchscreen then you can opt for the RCD330G. It has a nice updated MIB interface similar to those from the MK7 Golf and like the RCN210 has all the essentials out of the box. The RCD 330G can be had for around £100 and uses the existing Quad Lock Canbus cable. If you don't need the CD Player and fancy the bigger screen (6.5" vs 5.0") then you can opt for the RCD 330G PLUS (6RD035187A) which is what I have now, expect an extra premium of £80 on top of the standard model for this version however.
*** UPDATE - Prices have dropped dramatically for the RCD 330 PLUS making them fantastic value for money. They now can be bought for around £80 including postage from AliExpress ***
[28th Jan 17] Apple iPhone owners rejoice! A new RCD 330 PLUS (6RD035187B) has been released with Apple CarPlay integration meaning you can use your iOS apps such as Apple Maps, iTunes music via the system. Prices have now dropped and can be had for around £115 all in.
[22nd Nov 17] In specific markets VW has released the R340G (6RF035187C), an updated version of the B variant RCD 330 with full App-Connect functionality (Android Auto as well as Mirrorlink and Apple CarPlay) making it the most complete OEM unit you can buy with full smartphone compatibility. One store have managed to load this firmware onto the 6RD035187B units and is available for around £180 ish at time of writing, considering the CarPlay only units was around this price 10 months ago I would wait until it settles down.
Lastly there is the RCD 310 but unless you want DAB on the cheap then it's a bit barmy to choose this where there are better options! Like the RCD510 it requires additional modules for the Bluetooth and Media interface like AUX and USB so not a good value option if you want these features!
Hope this helps!
