The name of a hurricane is retired if it is especially deadly or damaging, and Hurricane Dennis was a very damaging hurricane. So now no new storms will be named Dennis (normally names are re-used...
The term Hurricane is a classification of intensity of a topical cyclone. In order to be considered a hurricane a storm must have maximum sustained winds of at least 74 mph.
No, Hurricane Opal was not the strongest hurricane by any means. Opal was a strong category 4 with 150 mph winds. The strongest hurricane on record was Hurricane Camille with winds of 190-200 mph.