Unfortunately, there can be no "the best" exercise, because any attempt to build your biceps will require you to do a lot of different exercises.
Bicep curls can be varied in so many ways, and there is no single exercise that can cover all of them.
grip supinated, pronated, or neutral
use dumbbells, a straight bar, or a curl bar
do them ...
You can't determine a difference in muscle size by measurements 12 hours apart. Any difference here is due to fluid levels (blood and water). Depending on how much activity you've had today, the amounts of blood in your muscles will vary. Depending on what you've ate and drank today, the amounts of water in your muscles and skin will vary. Best to measure ...