Hi There, does anyone know the best sprocket ratio for the 3EN2? I need to change the old ones and i was thinking of purchasing Renthal replacement sprockets.

depends if you are going to go the 520 conversion or stay as stock. think the stock is 19/55?? in 428pitch
or in 520 i've got a 14/46 that seems a good compromise of acceleration and top end.

Yes i would consider going the 520 conversion way, I have recently fitted the 3TJ Downpipes which are slightly longer than the originals, which the knock on effect is a slightly more reduced lower end power & higher top end performance. So i need to sort the ratio out to bring back more mid range power.

Give B+C Express a ring, they'll sort out the new sprokets sizes for you for the 520 convertion. Usually dropping 1 tooth on the front is roughly equal to going up 3 teeth on the rear, this will lower the gearing so you get better acceleration at the cost of a little top speed and fuel consumption.
I'd say to go up on the rear, that will shorten the wheel base a little and not give the chain a hard time going around a smaller front sprocket.