When I'm selecting strawberries at the market or farmer's stand, what should I look for to indicate the most flavorful berries?  I don't need the strawberries to keep; I'll be using them immediately.   
Smell. Really, this is the most reliable criterion for practically any fruit.   Flavor contains both taste and smell. For a strawberry, you want a fragrant smell together with enough sweetness. For both, the berry has to be ripe enough. If it was picked underripe, it won't smell good enough yet, and it will also be hard and sour. If it was picked long ago and is not fresh any more, it will have lost the more volatile components of its fragrance. So ripe, fresh strawberries smell great. I have had occasions when I entered a supermarket to quickly buy one thing, aimed at the correct aisle, but when the smell of good strawberries reached me near the produce, I turned and added a pack of them. I have never been disappointed with such strawberries. Also, if you smell the slightest hint of mold, fermentation, or foulness, you know they may not keep even one night.   Another sign is that a ripe strawberry will be red through and through. A strawberry picked underripe will be white or even slightly greenish at the top. It doesn't taste good then. But this is a negative sign, because not all red strawberries taste good.   Don't ever go by shape. The tastiest sort of strawberries my grandparents grew produced ugly, lumpy strawberries of a light, slightly orange color. They also had a few rows of a sort which produced perfectly conical, deep red strawberries, they looked like an advertisement - but they were hard and dry, and didn't have much aroma. Probably, there are some strawberries which both look and taste good - just don't think that looks or color predict a good strawberry, because they are independent.   And of course, any strawberries which have visible mold or fouled spots are not good any more. You can usually also recognize overripened strawberries in their appearance, but I don't know how to describe it well. They just look old.  