My order of preference:

Nippon-Yasan - I know it was hard to fit them in the poll but I've had impeccable experiences with them. Pricing is generally the best of any of the Japanese sites. Can combine pre orders easily. No upfront payment on preorders, able to use Paypal. Quick to reply to email questions, even when I'm asking about an upcoming pre-order like Thundercracker and Defensor. As most people by now know I'm quite fussy on packing and I've had 4 separate packages arrive this year, all with great care taken in packing.

Robot Kingdom - Can be pricey so generally only use when it's a hard to find or when I was in catch up mode on Masterpiece. Have to pay upfront which is a bugger but happy with packing and overall experience. Also answered my recent question on upcoming Thundercracker Masterpiece.

BBTS - have ordered a few Masterpieces when I was in catch up mode. Great customers service interaction with Joel, very flexible in allowing you to cancel pre orders. Strong USD means I'm likely not purchasing again from them for a while.

Ebay - depends on the seller but generally a good experience.

Premium Collectables - have a couple Trypticons arriving today as my first order, also have Thundercracker on pre-order. Pricing is very good, communication on the boards here seems to be better of late. A shame that there is no Paypal option available given the need to prepay up to 6 months in advance. Seems receptive to feedback about packaging options. Will reserve further judgement until I've had a couple packages arrive.