Hulk would take this relatively easily. Oh, there'll be a fight for sure, but Hulk is simply the greater monster. Stronger, has only been killed a couple times, fast enough to fight Thor, Sentry, and Gladiator, mad HF that even fixes disembowelment, broken necks, you name it, and fighting skills to contend with Thor who is inarguably the best warrior there is.
Grundy soloing the JLA is pretty rad, but it kinda screams "not so impressive" when they have the numbers and tactics and powers and still lose to one guy. Whatever they didn't do to stop Grundy, there's no reason why they'd think to do it to Hulk, so Hulk could probably down the JLA as well. Like, Wonder Woman suddenly realizes she has an unbreakable lasso, Flash realizes he could do the IMP, and Superman remembers punching out Darkseid? Take the issues where Grundy is soloing the JLA, look at everything they forget they can do to end it, and you could basically replace Grundy with Hulk and have it go the same damn way. And don't give me the J'onn crap. There's no reason his TP should work on Hulk. Hulk's got a thousand personalities locked up and he's batsh*t. If J'onn has trouble with Joker, he'd have trouble with Hulk.
Grundy is dead meat again.
As a side-note, if the JLA did what they're really capable of, they should be able to down anyone. Be it Hulk, Grundy, Sentry, or maybe even Thor.