The Superhero Database Classification number, or SHDB Class, is a number that represents the overall 'power' of a character. All traits of a character are used for calculating the Classification.
What it DOESN'T mean
This doesn't mean that a higher class would always beat a lower class character. But the bigger the difference in Class is, the more obvious it is who'll win in a fight.
How is this calculated
( INT^1.3 + (STR*0.5 )^2 + (SPE*0.5)^2 + DUR^1.6 + (POW + (SPS*SPL))^2 + COM^1.8 ) ^ TIER
Super Power Score and Level
Every Super Power has a score (SPS) that is used to calculate the Class. Each Super Power also has 3 levels (SPL). The level is set when connecting that Super Power to a character. The level determines the final score, of the Super Power, being used in the calculation.
My money is on Hulk, I don't think Flash has the tools to effectively put the Hulk down, plus Hulk will just get angrier and angrier. This fight kind of goes on forever because Flash can't really hurt Hulk and Hulk will have difficulty catching him but I think Hulk can react and tag him as he's tagged and kept up with Thor and Silver Surfer who transcend space-time (immeasurable speed), and Hulk punched the concept of time which grants him immeasurable speed.
Hulk mid-high diffs
Hulk fights, keeps up with, and regularly tags people like Silver Surfer and Thor who transcend time and have immeasurable speed themselves. Granted it won't be easy regardless