I love Decantshop.com. Their customer service is awesome, they don't gip you on juice, and they ship fast. I can't say enough good things about them.
I hate The Perfumed Court. They take forrrrrevvvvverrrr to ship, their sample sizes tell you how large the vial is, not low much juice you get, and their customer service is terrible... but their selection is tough to beat, so I still end up going there every now and then, but I avoid them if I can.
For Amouage, go to Parfums Raffy. 2.5ml samples with sprayers for $6. Killer!
For Le Labo, order directly from them. They're awesome about samples.
If I'm not mistaken, BN member 'Buysblind' recommended Perfumes For A Buck in a thread yesterday, but I haven't tried that one yet.
I look forward to seeing where others recommend.