Pistachio by Bakd. They have other flavors I really like, but that one had a synthetic, bitter awful taste... so bad that I bought another bottle thinking the first one was bad! Wasn't. Just bleh. But hey, taste is subjective and I know people who love that flavor...
Also, after years of jonesing to try Oat Drips flavors, there's some sort of underlying note to all of the flavors that turned me off. That said, it did push me to make my own Mungbean mix, which has been a favorite!