Well, my all-time fave is of course Godzilla. If I had to choose another I'd maybe go with King Kong as he evokes some sympathy as well as 'shock and awe'. For monsters from books I'd have to go with Smaug. He was my first love
and the reason I got into dragons.
Honorable mention to Mothra because he's so improbable as a monster but also has the coolest 'accomplices', the singing miniature twins.