Gosh! There are so many good ones to choose from these days. Others in this thread have mentioned most of the ones I've heard great things about. Also Viking Vapor and Vapor Station get strong
reviews.
I've just recently tried Alien Visions' Boba's Bounty, Flue-Cured, and Havana. All are very good, but need higher voltage or lower resistance. They are quite thick and smother my regular attys on 3.7V.
Read/view the detailed reviews of specific flavors and suppliers (but remember that the reviewer's taste buds are not yours). And check out candre23's sortable
cost per ml chart. I suggest sorting by "Large bottle" or "Sample bottle" to quickly spot the least expensive. [Clarification: The chart's elegant simplicity necessarily shortchanges the price edge of Tasty Vapor. Geoff's $15 120-ml 0mg "Doublers" combined with the $15 30-ml 48mg "E-Liquid" in the same flavor drive the net cost way down.]