Let's get ready to rumble!
For starters, stats: Ant-Man: height - reducible to half inch. Ant-size weight - 5 milligrams, give or take a smidge. The Hulk: 7'6 tall. Weight alternately listed at 900 lbs., 1,040– 1,400 lbs., and 1,150 lbs. (Does Hulk feel the need to lie about his weight? Is this an insecurity Ant-Man can take advantage of?) These noticeable height and weight disparities might seem to give Hulk the advantage - but consider their fighting styles: Hulk is a brawler, not a precision fighter. If Ant-Man can get inside Hulk's swinging range - a 1/4 inch away, for instance - the Hulk may have a hard time hitting him at all. Advantage: unclear.
Powers: Ant-Man - Changing size from normal human height, to ant-sized, while still having the strength of a normal man. Telepathic communication with other insects (range 1 mile). Good with electronics. Helmet also holds an additional air supply, so Ant-Man can breathe under-water.
Hulk: Hulk too can breathe underwater.
Hulk strong, jump far, regenerate. Hulk clap hands, make giant shockwave outward.
Ant-Man has the regular weakness of a regular guy. Hulk has pretty much no weakness - though boredom may put him to sleep, turn him back to Bruce Banner. (But this is not a “Bruce Banner vs Ant-Man” thread - which would likely put us to sleep too.) Being strong, jumping far, clapping shockwaves, breathing underwater - none of these things would automatically help Hulk if Ant-Man is hiding in his ear, or crawling around inside his torn shorts. If this was the case, Hulk wouldn’t likely be unable to block any of Ant-Man's swings. Ant-Man could land thousands of punches to Hulk's one. Then again - none of Ant-Man's punches would do anything. In a fight with Ant-Man, Hulk would swing away tirelessly, getting more and more frustrated, his anger building. With time and endurance on Hulk's side, Hulk would eventually connect to Ant-Man that one time. Yeah, that wouldn't be pretty. Final round to Hulk.
(Grape - meet Hammer. Metaphor not to scale: Hulk fist bigger.)
VERDICT: HULK SMASH!
(Note: If Hulk accidentally sits on Ant-Man before fight - this still counts as a win.)