My top 2 attributes are intelligence and passing accuracy. One decides when and where to throw and the other gets it there.
The other 2 blue bars help but they aren't absolutely required unless you have them under 50 or something then things can start getting weird.
Passing strength is important because it gets the ball there quick which reduces picks and pass knockdowns. Under 50 even smart and accurate QBs start throwing picks a lot because the D is as fast as the ball(small exaggeration).
Pass release to me is very important in this sack happy era. Pass release dictates how quick the receiver throws the ball when he decides to do so. This means less time for him to be sacked. It also means open receivers get the ball sooner.
Scramble is useful to prevent sacks too. It's basically your pocket awareness.
Hard count is a luxury. It's useful but not reliable enough to count on in a game plan. You will get more offsides penalties and it can destroy a very low discipline defensive line (30s) singlehandedly.
Last is speed and acceleration. It's not particularly important to me. Scramble is much more useful in my opinion as it picks better lines and get you out of trouble and follow your blocking. That said if your high speed QB picks right it could go for a 70+ TD.