well, don't keep us in suspense!
I'm only providing the name(s) because you asked.
I use NYAquatic.com for 95% of my online fish purchases. Mike and Leah are amazing! Mike has called me several times to apologize for any issue with my order or to Facetime with me so I can select the specific fish I want when there is a meaningful difference between items in inventory. Their live sales are epic, and you can watch them feed each fish and request a specific fish get fed again, so you can select the specific fish you want while paying half the price others charge for a blind, sight-unseen fish.
I've been very impressed with them.
Of the online retailers I've used, here is how I would rank them.
1. NYAquatic (best price, best selection, high quality, and unbeatable customer service/experience)
2. Aquarium Arts Online (OK prices, good selection, good quality fish, poor customer service, elite job with packing and optimizing shipping costs)
3. Violet Aquarium (HUGE WYSIWYG selection without a meaningful premium, mediocre customer service {they had poor communication around shipping and didn't deliver one of my items, but charged me, it was a plant, so I didn't feel like going through the hassle over a plant.}, good quality fish)
4. LiveAquaria (Very good selection, poor fish quality, OK pricing, terrible customer service)
5. SaltwaterFish.com (GROSSLY OVERPRICED!)
I've also used Biota, but they are a specialized retailer, so I don't include them in this list. They did have a nice customer experience. Unfortunate for me, I didn't QT the fish because they were captive bred from a very high quality retailer, it cause a velvet outbreak and several thousand dollars down the drain in 2.5 days. I blame myself for not doing a full medicated QT. It was an expensive lesson to learn.
I'm not saying I've never had a DOA or received a sick fish, but those experiences were/are rare for them, and when they occur, Mike and Leah are quick to correct the issue in a hassle-free manner.