Cold nuclear fusion
Women. Ages 18-35. I’d love to get me some real hands-on experience. 
But I can pick up your subject on the side. I doubt you can do the same for mine. 
- What subject would you choose to become the world’s greatest expert in? *
Alyson Hannigan.
I realize it would take years of close examination, and countless hours of hands-on study, but I am ready to begin.
Barring that blatant fantasy - automotive internal combustion engines and drivetrains.
I’m an automotive mechanical geek. A very lonely one.
Unless he’s started wearing a ‘hoody’ and hanging about the streets drinking ‘Wicked’, kicking passing cats and generally being a public nuisance, I doubt our paths would cross. But if they did, I’d obviously let him off with a stern warning.
Guitars.
Martial arts. If I didn’t have to have a job I’d be a freakin’ Swordmaster of Ginaz.
Either
Anaerobic Digestion, especially tweaking the consortia into digesting wastes,
or
Humorous Writing Through History.
No - he’d maybe let you off with one 
Chuck would still round-kick you to the moon and back. Just as ivan astikov
Chuck Norris defies all champions (no matter what they’re the champion of) and wins.
But what if Chuck were teaching the class, with the express purpose of creating a badass badassier than even he is? Surely you’re not suggesting that Chuck could FAIL.
Especially as he’s standing right over there, round-kicking Galactus into submission.
You just made my head explode - that just can’t be.
Technical Writing.
Really.
Then I would use my new-found powers for good, and write the perfect instruction manual for technical writing.
And never again have to slog through five manufacturer’s manuals to write a useable SOP.
-
somebody's has been spending way to much time reading manuals recently *
Brewmaster or master distiller.
Fluvial morphology.
Just kidding! Actually I’d like to learn as much as I could about dam failures, and I’d love to add to the body of knowledge about the subject through research. I’m probably too old now though. 
Visual effects.
Right now I have a C average, though.
Animal behavior.
Human pathophysiology.
Right now I’m into waiting for caterpillars…this is the first ever year I’ve not seen any on the thousands of plants I grow every year. No monarchs, a few tiger swallowtails, a few Black swallowtails… no caterpillars, at all. I’m becoming an expert at waiting for caterpillars.
Oops, you said “choose to”…Damn.
I’ve always wanted to be an expert witness, or “noted” in a field. Maybe some aspect of forensic science. Toxicology? Animal behavior?
Music, specifically, composition.