The Kona Velvet MilkShake is my favortite vape....period. I switch out flavors a few times a day, my favorites being mainly coffee vapes. If I had to choose an all day flavor, it likely would be the Kona Velvet Shake. It's expensive, but it is worth it to me.
Coffee, from Copper Creek, is an excellent flavor. A nice black coffee, I prefer adding sweetness and cream to this coffee flavor with Sweet Cream E-Liquid from My Freedom Smokes (add to taste).
Others I recommend are Organic Double Espresso and Organic Kona Cream from Vaperite. Vaperite is currently offering 20% off with the coupon code "20 OFF" at checkout. They offer sampler packs also, for low risk flavor testing. These flavors have a wonderful coffee aroma upon opening the lid and in the vapor.
Virgin Vapors, in addition to the above KVM, has sampler packs available. The Dark Roasted Turkish Coffee is very good, too. Although I haven't tried it yet, I've read good comments about Annette's Espresso flavor.
Backwoods Brew's Espresso is good as are a couple of coffee flavors from Dark City Vapor.
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A word of advice. All of the above are dark-colored juices, so expect to need to replace cartos more frequently as the polyfil becomes clogged up with the unvapable particles in the juice. These all will benefit from a week or two of steeping time, too.