Im going to say Goku, i hope it upsets certain ppl.
Goku has to have lightning fast reflexes to pass Popo's test as a child, in which before this he was a casual machine gun bullet timer. Z senshi at the very beginning of Z can dodge ki blasts (which have been clocked FTL a number of times) even at ridiculously close ranges. As a child when he was lightning quick, his power level was about 150, when he first turns ssj, his power level is 150 million. (in daizenshu) Based on snake way speed feats, the indication is that powerlevel and speed have an exponential relationship rather than a linear one.
@Sonata: There seems to be a huge discrepancy between the speed in the first scan, to the second scan. The first scan I believe he is referring to the speed at which he thinks (time period he can count) in the second scan it is travelling distance. Still in combat reaction speed I give the edge to Goku.
@hitsusatsu11: Yes the first scan shows his reaction time,and the second one shows his travel speed. When a powered up Flash fought against Zoom they fought around the world a dozen times in less than a second. Goku has never shown the reaction time off an Attosecond, there are multiple instances where he was simply not fast enough. Like Majin Vegeta destroying the Stadium.
here is another scan that shows light is having difficulty keeping up with the flash.