I've used many of these and have been satisfied by all. As long as you stick to reputable suppliers its hard to go wrong. Just installed a Dayco HB belt and it seems to whine. Have heard this will dissipate with time. Other than that the kit was complete. Very satisfied.Hard to name a "Best" in this (or any related) category...
Most of the well-known suppliers have gotten their reputation from doing right by their customers... They know if they don't "go the extra mile", their competitors will...
In no particular order:
www.dieselgeek.com
www.metalmanparts.com
www.boraparts.com
www.idparts.com
www.worldimpex.com
www.ecstuning.com
and for the Canadians:
www3.ns.sympatico.ca/roseland/VWPartsA4.htm
www.precisiontuning.ca
(and I'm sure I left someone out... apologies...)
Yuri
(I've dealt with 6 of the above suppliers and have received top-notch service from all...)