As both Flash & Hulk are comic book characters that "inhabit" different universes its really not possible to proclaim one as a conclusive winner. Each have powers that bear no resemblance to our currently accepted perception of reality.
However of the two the Hulk is, in my opinion, the more plausible character as the source of his powers are better explained and easier to imagine. He is not a"normal" human - his genetic/physical makeup has been fundamentally altered. His main powers being incredible durability, almost instant regeneration (greatly accelerated healing) and incredible strength which increases as his fury mounts.
Unfortunately most people confuse strength with punch/strike power - i.e. the stronger you are the harder you can punch. Punch/strike power is a function of speed, weight. timing, technique & co-ordination. Strength is the ability to either lift/move heavy objects or & overpower opponents with muscular force. For the Hulk to shatter say a mountain he must be either moving at super-speed or discharging some kinda energy blast on impact.
The writers/creators of the Flash based his powers at least in part on their misunderstanding of Einstein's Theory of Relativity. Instead of just making him a mutant with a greatly enhanced genetic/physical makeup they left him as a "normal" human. To explain his powers the "Speed-Force" concept was invented in the 1990's.
The Speed Force is a vaguely defined extra-dimensional energy field that supposedly allows the Flash to operate at/near light speed. (plus a bunch of other powers) This is nothing short of an incredible cop out as there is no explanation given as to why this "Energy Field" would effectively envelop the individuals that it has. How or why would the Speed Force be attracted by or to one person and not another. Can anyone tap into the "Speed Force"? The process of the Speed Force interacting with the Flash is just not explained.
Then just to confuse things even further they invent the "infinite mass" punch. The infinite mass of Relativity does not mean infinitely heavy as in weight, it means mass as in the interchangeability of mass and energy. (E=MC2) Furthermore as a mass-energy form (object/body) gains speed by processing energy it changes state.
Like ice (physical/tangible) becomes water (less solid/liquid) and finally water vapour(gas/intangible). Sun light travels at the speed of light and strikes/interacts with many other mass-energy forms including the surface of my skin without delivering a "infinite mass" punch. The Flash travelling at or near light speed would not be tangible or solid & therefore could not punch anything. Hell even if the Flash were a lot slower & remained "physical", punching anything at say 65,000 mph (avg speed of the earth's orbit), would require him to have incredible durability or he would be destroyed by the impact of his own blow.
The other idiotic power is the notion of stealing/lending speed. Firstly speed is not a thing, it is a complex process of multiple interactions that involve changing one form of energy into another. Speed just cannot be stolen or lent, only the process that produces movement (speed) can be disrupted - but this would take "magical" intent (sorcery).
There are some other weird powers which belong to the realm of magic rather than that of a speedster but the above two are the worst.
With the Flash get rid of the Speed Force BS and make him into a mutant with speed, strength and corresponding durability or make him into a goddamn wizard. For the Hulk recognise that incredible punch/strike power requires far more than just brute strength or make him into a grappler that rips his opponents apart.
There is a vast difference between ill conceived nonsense concepts that cannot be related to by anyone capable of independent thought, and well imagined plausible characters even if they are unrealistic. Imagination should not be confused with meaningless concepts and word games designed to bamboozle the gullible.