Elizabeth I was probably the most successful monarch in English history. Her skill in foreign and domestic matters have justifiably led to historians designating the period of her reign as the "Age of Elizabeth I.
Elizabeth was schooled in Latin and Greek, yet also fluent in English vernacular, which allowed her to speak to sailors in language which they could understand. She was very skillful in working with Parliament to whom she often made brilliant speeches, and frequently made herself visible to her subjects, normally dressed radiantly. She wisely avoided the religious excesses of her father, Henry VIII and her half sister, Mary I, by refusing to accept the title of Supreme Head of the Church, an honor she said belonged to God alone. Under her leadership, Parliament passed an Act of Uniformity which added sufficient elements of Catholicism that many Catholics could worship in the Anglican Church without damage to their consciences. This satisfied many, though not all, religious issues of the day.
Aside from her tact and diplomacy, Elizabeth did not hesitate to act deliberately and forcefully when she saw the need. She signed the Death Warrant of her cousin and likely heir, Mary Queen of Scots when the latter was involved in plot of overthrow Elizabeth. Similarly, when Philip II, in a last ditch attempt to restore Catholicism as England's state religion launched the Armada against England, Elizabeth appeared before her troops in full body armor, and promised to lead them herself into battle should the Spanish land on English soil. She told her army, "I know I have but the body of a weak and feeble woman, but I have the heart and stomach of a King; and of a King of England too."
These accomplishments, plus her support of such gifted writers of her age as Shakespeare, Marlowe, and Spenser aptly justify the title, Age of Elizabeth. Two excellent sources on her life are The Lady Elizabeth, and Elizabeth I, by Alison Weir, and excellent Tudor historian.
It would be extremely difficult to argue that Elizabeth I was not a successful monarch. After all, she reigned for 44 years during a time in which her country became stronger in terms of military might and empire and during which its culture flourished. Her reign soon came to be seen as a golden age for her country. This seems to be the very definition of success as a monarch.
When Elizabeth came to power, England was not one of the most important countries in Europe. Spain and France were much more powerful countries. England itself was not particularly stable, with conflict between Catholics and Protestants (as well as purely political conflicts) lessening its stability. By the time Elizabeth died, England was probably the most powerful country in Europe and her reign had settled the issue of religion in England.
The stability she brought to the country led to a flourishing of arts. Her reign is synonymous with Shakespeare, the greatest of poets and playwrights. This in itself is a major accomplishment. For these reasons, Elizabeth I has to be seen as a successful monarch.