She was a formidable and talented woman but Charles had good reason to be concerned. All of her financial wizardry had not been enough to keep the family on solid financial ground as Prince Florestan was as liberal in his spending as he was in his political opinions. Prince Charles III wanted to get out from under the Savoys and return Monaco to the old protectorate with France. He also wanted to restore the Grimaldis to their place at court in Paris, having sorely felt the lack of attention from King Louis XVIII, Charles X and Louis Philippe. He hoped Emperor Napoleon III would be more friendly. However, all of these ambitions would cost money and that was something Monaco nor the House of Grimaldi had in abundance. The tensions were also growing worse with Turin. It was left to Prince Charles III to sort all of these issues out and to steer through the complex world of Franco-Sardinian relations. In the end, it cost Monaco the towns of Menton and Roquebrune but, in the end, in every other way Prince Charles III was to see all of his goals accomplished and would usher in a new era for the Principality of Monaco.
I - The first is the first, that is Prince Honore II, who, as we know, was the first Grimaldi to be formally titled “Prince of Monaco”. This guy was something. Even as a baby his life was in danger as the assassins of Lord Hercules I ransacked the palace trying to find him to kill him as well. He became a very astute statesman, never had any marital problems from all I have read and his surprise attack which took Monaco out of the Spanish camp and into the French one shows that he was a clever diplomat as well as a masterful tactician. He obtained a much better position for his country, beautified the place and in every way left it better than he had found it. His people adored him for his wisdom, kindness and courageous leadership. The Spanish were not too happy with him but most every other account of his life and reign speaks of him only in the most complimentary terms. Monaco was his life, he devoted himself to it and to making it better. In that, he fully succeeded. 
II - Prince Antoine I, unfortunately had some setbacks in his reign. He was obliged to recognize the Duke of Savoy as overlord of part of his realm, his marriage to Marie de Lorraine was a disaster and so on. However, he inherited a nearly bankrupt state and yet managed, through immense personal sacrifice, to find enough money to improve the defenses of Monaco when the country was under threat. He was also a great patron of art and music and was the first to make Monaco a center of such creative talent, which might have surprised some since the Prince was also a very heroic veteran soldier. The people of Monaco adored him because of the concern he showed for their safety and the sacrifices he was willing to make to protect them and their property. He could be stern and strict but such a firm hand was called for and he not only secured and improved Monaco but pulled the country back from the brink of economic ruin and left it better than he found it. 
III - Prince Charles III, in many ways, was the monarch who put Monaco on the map. After losing the vast majority of the territory of the principality to France (the ceding of Menton and Roquebrune) it was Prince Charles III who presided over the economic renovation of Monaco into the holiday hot spot of Europe. The country had fallen on hard times since the whole trauma of the French Revolution but under Charles III the situation finally stabilized. The Italians were dealt with once and for all, the relationship with France was put firmly in order and a new era of economic independence came with the establishment of Monte Carlo casino and the rise of the resort industry. Much of this was also due to the wife of Charles III, Princess Antoinette, and his mother Princess Maria Caroline but it was Charles who was ultimately the one responsible as Sovereign Prince. Monaco adopted an official national flag, national anthem, the Order of St Charles was established and diplomatic relations were established with a number of countries all during the reign of Prince Charles III. 
IV - Prince Louis II might not be included on every list of great Princes of Monaco but he is one of my favorites. He embarked on a military career in the French army and took his service very seriously. Many said he would have been happier as a soldier than as a prince and he was a very good soldier. During World War I he displayed great skill and valor, winning numerous decorations and promotions. He was proud of his service and was often seen in uniform. During World War II when Monaco was occupied by the Axis forces, Prince Louis II used his police connections to ensure that Jews were able to escape before they could be arrested by the Nazis. He took a major risk in doing this and had to walk a thin line during those years. As Prince of Monaco he took the first steps toward a greater diversity in the tourism industry, making Monaco a center of football and racing as well as gambling. It was the steady leadership of Louis II that saw Monaco through its darkest period since the Revolution. 
V - Prince Rainier III, I firmly believe, will go down in history as one of the greatest Monegasque monarchs of all time. His extensive list of accomplishments are nothing short of exemplary. A strong monarch but also a forward-thinking man he brought Monaco out of the devastation of World War II and made it, once again, the most famous spot on the Riviera. His marriage to Princess Grace made Monaco a household word around the globe and he was a devoted husband and father. He diversified the economy to get away from the over-reliance on gaming and made Monaco a tourist destination that everyone could enjoy, not just the super-rich. He dealt firmly with any threats to the public good, reworked the constitution and strengthened the police to keep organized crime out of his country. Monaco became a center for business under his rule, the country expanded as land was reclaimed from the sea and a supreme court was established. His economic diversity programs brought a wave of unprecedented wealth and prosperity to the country. Although not often recognized, he was also a very humorous man and a very religious man and was awarded the Order of the Golden Spur by the Pope. At the time of his death in 2005 he was the second-longest reigning monarch in the world. 
Honorable Mention: It would be premature to pass judgement on the reign of Prince Albert II, however, some mention should be made of the admirable job he has done so far. He has made Monaco a center of environmental study (particularly concerning the oceans of the world), uprooted criminal elements trying to take root in the country and opened a new era of transparency that has taken Monaco off the list of 'tax havens' and "Morality, honesty and ethics" has been his motto. The economy has done well, even going through the current world financial storm relatively unscathed and relations with France have been greatly improved as well as managing to increase the effective political independence of the country. So far, so good.
