I'm still a bit new at this, but I've been especially impressed with:
John at cinemagraphics (eBay) - lots of personal communication, high quality LCs, reasonable prices, and a few extra surprise cards included in my orders. Personal touch, and a great guy.
Stefan (slidemen) (eBay) - Great communication, high quality mostly German card sets that I've haven't found elsewhere on eBay. Great prices on mint full sets, and a personal "thank you" for each order.
toddfeiertag (eBay) - Outstanding quality, and the most bulletproof packaging I've seen (double cardboard taped with finesse, in peanuts, in a large box). Wow.