In the end Superman doesn't have the speed force, so (Post Crisis, at least) he'll never be faster then someone who has it and can take full advantage of it. Now Pre-Crisis Superman, who can fly fast enough to fly through time as easily as he could go for a walk down the street, he's probably faster then The Flash. But Post Crisis (and even New 52 and Rebirth) Superman I've never seen able to run (or fly) faster then light/through time/dimensional barriers, or any of the other speed force things we've seen Flash do.
So, I'd say that modern Superman comes in second in speed to the experienced Speed Force users in their prime, which is still faster then the vast majority of everyone who has ever lived in the whole DC Universe. I've always thought that Superman is such an effective hero because of the variety of his powers anyway, not that he's the absolute best at literally everything he can do (not counting Pre Crisis Superman obviously, who was literally the best at anything and everything the writers could think of).