Of the different brands I’ve tried, the three best tasting (in my opinion) are:
Cytosport Muscle Milk - Chocolate
ON Pro-Complex - Rocky Road
BSN Syntha-6 - Vanilla Ice Cream
To be fair, I’ve only mixed the Syntha-6 in heavy cream, which enhances the taste of most powders, anyway.
But, it’s the only vanilla-flavored powder I’ve ever liked at all.