I'm partial to Atlantic and Cigar Auctioneer for box purchases. Got turned onto Cigar Page in the past few months as well for their 5 and 10 pack deals. Holt's has a lot of excellent deals on Oliva, Villiger, AJ Fernandez in their weekly and daily sales. The great Smoking Pipes might not have the best deals but they have a fine, very well-curated selection, sell singles and are a site sponsor here so I have to give them some love, too! They put a lot of care into their descriptions and written copy which I think is valuable for branching out and trying new things.
I don't use them because they have to charge me California tobacco taxes since we're both located here in the Golden State, but Small Batch is awesome for carrying a lot of smaller, "boutique" brands. If you're outside of California, you don't have to pay the CA tax, I believe, and everyone raves about them. Similar to SP, they put a lot of attention on offering a well-curated selection and provide great descriptions and content which makes it easy to learn about their offerings.
Some of the biggest sites drive me batshit by obscuring which cigars are house cigars or mixed filler or have inaccurate descriptions, leaf details, etc. You can get good deals but I suggest cross a referencing with other sites so you can be sure you know what you're getting.