Here's what Pauline Kael had to say about Ricardo Montalban in Star Trek II: The Wrath of Khan:

Montalban is unquestionably a star in The Wrath of Khan (and his grand manner seems to send a little electric charge through Shatner). As a graying superman who, when foiled, cries out to Kirk, "From Hell's heart I stab at thee!," Montalban may be the most romantic smoothie of all sci-fi villains....And that great chest of Montalban's is reassuring—he looks like an Inca priest—and he's still champing at the bit, eager to act: he plays his villainy to the hilt, smiling grimly as he does the dirty....You know how you always want to laugh at the flourishes that punctuate the end of a flamenco dance and the dancers don't let you? Montalban does. His bravado is grandly comic....This man, who believes that his search for vengeance is like Ahab's, makes poor pompous Kirk even more self-conscious. Kirk is Khan's white whale, and he knows he can't live up to it—he's not worth of Khan's wrath.

But then again, who would be?