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:
and for the Canadians:
(and I'm sure I left someone out... apologies...)
(I've dealt with 6 of the above suppliers and have received top-notch service from all...)