Prussia became a kingdom relatively late, in 1701 to be precise - much later than the old kingdoms of Western and Central Europe. Some of its kings were very good to great, some others pretty bad to abysmal, but barely anyone came down to "just average".
Frederick William (Friedrich Wilhelm), the Great Elector of Brandenburg (1640-88). Came to power while his country was occupied by Swedish troops. Acquired most of Pomerania and some other lands in the Westphalian Peace after the Thirty Years' War, making Brandenburg the second-biggest German state by area. Worked hard to build up the war-ravage and mismanaged (by his father) country. Even acquired some colonies in Africa, although they were lost again later. Invited the Huguenots who were kicked out from France by Louis XIV. With his reorganized army he scored some successes that made people take notice, such as the victory of Fehrbellin (1675) against the Swedes, but also suffered reverses. However, his participation in the First Northern (or Swedish-Polish) War had important consequences, as the Peace of Oliva (1660) removed the suzerainty of the Polish King over the Duchy of Prussia, which meant that in East Prussia Frederick William and his successors now disposed of a sizeable territory in which they were absolute rulers, subject neither to the Holy Roman Emperor nor the King of Poland.
Elector Frederick (Friedrich) III, later King Frederick I (1688-1713; king since 1701). Not pretty to look at, as he had a bit of a hump. Technically, he wasn't king of Prussia, but king in Prussia, i. e. that part of his territories that lay outside the Holy Roman Empire; however, in common parlance "Prussia" (or "the Prussian States") replaced "Brandenburg" as the name for the monarchy in total. For his Awesome Moment of Crowning he spent the whole income of the state during three years. Also, to get the acquiescence of the Emperor to his new status, he had to support the Habsburgs heavily during the War of the Spanish Succession that began shortly after. The new Royal Prussian Army soon found itself fighting pitched battles as far afield as Turine in Italy and Malplaquet in France. Frederick is said to have aped Louis XIV to the extent that he took an official mistress even though he dearly loved his wife, and turned a blind eye to the rampant corruption in his administration. His reign also saw the foundation of the Berlin Academy of Sciences, brought on the way by Queen Charlotte and the philosopher and mathematician Leibniz. When Frederick died, the state was almost bankrupt.
Frederick William I, the "Soldier King" (1713-40). Built up a strong army, up to the point where Mirabeau would say "Prussia is an army with a state!" Still, he only went to war once during his reign. Introduced compulsory education and a centralized administration. Didn't care at all for luxuries, almost lived an ascetic life, thus managed to get the financial troubles inherited from his father under control. Still was very much a tyrant, which caused his son (the very Frederick The Great, below) and his best friend to try to flee to France, which caused the latter to be executed. The prince himself only escaped this fate because the Emperor himself intervened.
Frederick The Great (1740-86) aka "Old Fritz". Has his own page. During his reign the title changed from King in Prussia to King of Prussia.
Frederick William II (1786-97), nicknamed "fat Wilhelm". Nephew of Frederick the Great. Had many affairs with women, some of which he made his morganatic wives. Sent his army to the Netherlands when the "Patriot" movement there tried to depose his brother-in-law, the Stadhouder. This almost bloodless campaign resulted in the immediate collapse of the "Patriots" and a resurgence of the Orangists. Later went to war against France when the revolutionary government declared war in all directions as a solution to its internal problems. The Prussian army in fact won several battles against the revolutionary armies (which led to a dangerous feeling of self-satisfaction afterwards), but in 1795 Frederick William concluded a separate peace with the French Republic to concentrate on the newly erupted conflict on his eastern border. The subsequent Second and Third Partitions of Poland took that state off the map.
Frederick William III (1797-1840). Rather like his namesake, the Soldier King, he loved the army but was extremely reluctant to go to war. His modest personal lifestyle and his "model" marriage to Luise of Mecklenburg-Strelitz (at the time one of the rare love-matches among royals), endeared him to his people, but his hesitancy and shyness proved a serious disadvantage opposite Napoleon Bonaparte. In the war of 1806/07 his army was heavily defeated by Imperial France and Prussia lost half its territory and population. This augured in the era of Prussian Reforms, which modernized the state, its administration and army and is associated with the names of Stein, Hardenberg, Scharnhorst, Boyen and Wilhelm von Humboldt. Serfdom was abolished, municipal self-government instituted, Jews became citizens and the old restrictions on the choice of profession were removed. In the popular image, Frederick William was not so much a ruler himself but let his competent ministers and generals do the job, how much influence he took himself is a matter of debate among historians. He resisted calls by nationalists to go to war against France again in 1809, which some saw as cowardice and others see as realism. When the Grande Armée was destroyed in Russia, he finally changed his mind; here he was in part influenced by General Yorck, who had taken the Prussian contingent of the Grande Armée out of the war by concluding the convention of Tauroggen with the Russians in late December 1812. At the beginning of the Wars of Liberation, the king instituted the Iron Cross on the birthday of Queen Luise, who had died in 1810. The preceding reforms of the Prussian Army enabled it to be expanded with recruits and volunteers from 42,000 to over a quarter million within months and it thus played a major part in bringing down Napoleon in the two wars of 1813-14 and 1815. After that Frederick William returned to his peace-loving ways and for instance forestalled a war by the monarchic powers against France when war appeared imminent after the July Revolution of 1830. However, Frederick William disappointed democrats and nationalists because the German Confederation which was formed at the Congress of Vienna was neither united nor liberal, let alone democratic, and within Prussia he never made good his 1815 promise to give the kingdom a constitution. From 1819 Prussia and Germany fell into a period of reaction and political stagnation, however in the sectors of economy (industrialization and the Deutscher Zollverein, a tariff union of Prussia and a growing number of other German states) and the military modernization marched on. Frederick William realised his pet project of uniting the Lutheran and Calvinist churches in Prussia, but only at the cost of some discontent and making a number of "Old Lutherans" leave the country. He also was a composer at the young age of ten.
Frederick William IV (1840-61), the "Romantic on the Royal Throne". Started to liberalize Prussia somewhat when he came to power. Decided to finish the cathedral of Cologne. But he managed to screw up big time during the revolution of 1848. When the Democrats set up a constitution for Prussia, he said that he didn't want "a piece of paper" between him and his people, and when he got the chance introduced a less liberal constitution of his own. One part of the parliamentarians in Frankfurt offered him the German emperor's crown, but he declined, not wanting a crown from the hands of the people (the fact that the diplomatic situation would have made that a suicidal act didn't exactly help either). Under his reign the Prussian army introduced the Pickelhaube, but also was re-equipped with breech-loading rifles. Frederick William IV was long believed to have gone demented late in his reign, but actually suffered from a succession of strokes that rendered him incapable of ruling from 1857, making his brother William the regent. Since Frederick William's marriage with Elizabeth of Bavaria was childless, he was succeeded by his brother.
Wilhelm (William) I (1861-88). Brother of Friedrich Wilhelm IV. Had become infamous during the failed German revolution of 1848, for ordering to shoot on demonstrating democrats. Was only the second king (after Frederick I) who had a proper coronation ceremony. His first years weren't that mentionable, but then he appointed Otto Von Bismarck Prussian prime minister. Then, during only seven years, Prussia and Austria defeated Denmark and took Schleswig-Holstein, then Prussia defeated Austria so they were free to found the North German Confederation, then defeated France, toppling indirectly Napoleon III, uniting all of Germany and taking Alsace-Lorraine, too. See also Imperial Germany. As Bismarck said, he didn't care who was king below him. (Or was it Wilhelm stating it wasn't easy being king below Bismarck? Whatever.) His grandson Wilhelm II tried to have Wilhelm I be called Wilhelm the Great, but that appelation did not survive the year 1918.
Frederick III (1888). Died after 99 days in office, because his incompetent doctor misdiagnosed his cancer. Some people wonder whether a longer reign of him would have helped democratizing and liberalizing Germany, let alone avoiding World War One and whatfollowed, but that's speculation for Alternate History.
Wilhelm II (1888-1918). Grandson of Queen Vicky. Had a crippled arm, because an incompetent doctor messed up during his difficult birth. Many people claim this is the reason why he was (Over) Compensating for Something, with his uniform fetish and building up the German navy (together with admiral Tirpitz), with had to tick off Britain. Also had a bad relationship with his mother, British-born Victoria, who was in favor of liberalism and democracy. Fired Bismarck after two years in office. Sometimes acted like a Jerk Jock, for example when he once slapped the Bulgarian czar on his butt. No, really! Once met Annie Oakley in her Wild West show and volunteered for a stunt that could have backfired - she would shoot off the tip of a cigar he held in his hand. Wanted "a place in the sun" for Germany. Infamously made his "Hun speech" during the boxer rebellion. (Making things easier for British propaganda in World War One.) With the "Daily Telegraph" affair, he was close to stepping down, but stayed at the end. Wasn't able to prevent World War One, despite being related to most European monarchs. After 1916, practically powerless because the generals Hindenburg and Ludendorff now ran the country. Stepped down in 1918 and went to exile in the Netherlands, in the city of Doorn. Spent most of his time there with woodcutting, killing thousands of trees. Wasn't too fond of The Nazis (and even less of the fact that some of his sons liked them), but hoped that they might help him to return Germany to the monarchy. Still congratulated Hitler for defeating France. Died in 1941.