Westworld’s MIB is in Host form with full abilities. Lost’s MIB can change into Smoke Monster only when on the island but otherwise remains with his powers.
Round 1: In the Delos parks.
Round 2: On the island.
Round 3. At Stonehenge. (UK)
Round 4: In Times Square while a gigantic squid is also attacking.
Round 5: Fighting on top of Mount Roraima.
Bonus round: Boy in White from ‘From’, vs both Man in Blacks, setting is Fromland/Fromville and he has all his abilities.
I never got around to watching Westworld, but if it’s alternatively the MIB (Men in Black) VS the Man in Black (Lost), the Man in Black will likely take the win, if only from attrition. However, if the MIB neuralyzer works on ol’ Smokey, potentially they could convince him into being human enough to be constrained. Personally, I don’t think the neuralyzer would work on him though, because with the Heart of the Island in tact the form he takes isn’t really fully human.