When reading books on Ottoman history, such as the Ottoman Centuries, I see the authors always characterize Suleiman as a laudable monarch, piling praises for how he led the Ottomans during the peak of its power.
However, when actually reading about his feats, I don't find Suleiman to be, in anyway, more than just a good monarch. There is nothing about him, in my opinion, that would make him any better than his predecessors. His campaign against Austria is a prime example. He was also quite vain, killed the most capable son he had, and was controlled by his lover Roxelana.
Maybe this is because I have not gone past the superficial part of Ottoman history. But what did Suleiman do that would qualify him as an amazing conqueror and an exceptional humanist, as many would claim?
It's true that many of Süleyman's achievements are largely invisible in the older historical narrative of Ottoman history. This now outdated framework for Ottoman history that you find in books like The Ottoman Centuries, often called the "Rise and Decline" paradigm, portrayed the Ottomans as constantly and gradually gaining in strength until the time of Süleyman I and thereafter constantly and gradually weakening. The period from Mehmed II (r. 1451-1481) to Süleyman I (r. 1520-1566) was portrayed as a sort of "Classical Age" in which the Ottoman Empire functioned in perfect harmony, until the "decline" afterwards. This makes it seem as though the real architect of Ottoman success was Mehmed II, and Süleyman just effortlessly ruled over the empire which his great-grandfather had created. In fact, the system created by Mehmed II was under a great deal of strain by the time Süleyman came to the throne, however because mentioning that would violate the principle of "before Süleyman the Ottomans were constantly getting stronger," this fact didn't get much recognition in the older histories.
Basically, by the time of Selim I (r. 1512-1520), the Ottoman state was ruled by an old guard of men who had risen to power in the time of Mehmed II and Bayezid II. While their positions had originally been designed with total subservience to the sultan in mind, as the decades passed they increasingly came to think of themselves as privileged members of (something resembling) a noble elite. Selim I was a forceful ruler, but his death put Süleyman in a terrible position in relation to the Ottoman elite: all of his viziers were experienced men, while Süleyman was young and was the first Ottoman sultan to take the throne without having had any prior military experience whatsoever - his father had left him behind to govern Istanbul during his various eastern campaigns. So Süleyman started with a very weak position and few avenues with which to utilize his power. He managed to flip this situation on its head by winning crucial military campaigns in Belgrade and Rhodes while playing the major viziers against one another. Over the course of the 1520s he removed the entire old guard from the top ranks and replaced them with men of his own making, most famously Pargalı İbrahim Pasha, a friend from his teenage years who he made into Grand Vizier. Thus Süleyman was responsible for reversing the change in the ethos of the Ottoman elite and placing himself firmly in a position of ultimate power.
But leaving that particular issue aside, Süleyman's reign was praiseworthy for a variety of reasons. I'll list some below:
And much more which I'm probably forgetting.
I should also note that things like this:
He was also quite vain, killed the most capable son he had, and was controlled by his lover Roxelana.
Are a direct result of reading old orientalist books like that written by Lord Kinross. Mustafa as the "capable son," Roxelana (Hürrem Sultan) as the "scheming harem woman." They're historical stereotypes, not real history. Of course it's not your fault for picking a bad book, but now you know - avoid Kinross at all costs.
For the latest analysis of Süleyman's reign:
Şahin, Kaya. Empire and Power in the Reign of Süleyman: Narrating the Sixteenth-Century Ottoman World. Cambridge: Cambridge University Press, 2013.
Pros & Cons of Suleiman:
Superpower
Pro: Ottoman Empire reached the peak of its power during his reign.
Con: Successful campaigns led by his father, Selim I, made Ottoman Empire the wealthiest/powerful state in the world.
Military
Pro: Magnificent victories like Battle of Mohács (wiping out Hungarian Army in couple of hours) and Siege of Rhodes (a tough nut to crack) shows us that he was fully capable of commanding armies.
Con: Disastrous campaigns like Siege of Vienna (not being able to install proper supply lines during WINTER) and everlasting Ottoman-Safavid Wars (not being able to gather intelligence. I mean, he couldn't find the fucking enemy) shows us that he was nowhere near his father (Selim I) as a commander.
Intellectuality
Pro: He was blessed with a bright mind and artistry. He was by far the greatest poet from Ottoman House (Ottoman literature was largely inspired by Persian poetry tradition. More than half of Ottoman Sultans were deeply involved in poetry)
Con: His intellectual capital was nowhere near his great grandfather Mehmed II, who was, for lack of a better word, a Renaissance Man, arguably the most colorful intellectual of his era.
Law
Pro: His title isn't the magnificent, but Kanuni (The Lawgiver). He issued non-Islamic laws which harmonized social life within the empire.
Con: You can find other Sultans who issued laws. Legal reforms and laws issued by Mehmed II (Kanunnames and Yasaknames) were as constitutive as Suleiman's.
Constantinople
Pro: His greatest legacy must be Constantinople. The city was in ruins after the sieges by Crusaders (1204) and Ottomans (1453). Suleiman spent an immeasurable amount of money to bring its former glory back. He invited artists from all around the world. He put thousands of artists on payroll through art communities (ehl-i hıref). Architects like Sinan, poets like Baki, calligraphers like Karahisari enjoyed a prosperous life under his patronage. He set the tone for 2 centuries which can be described as The Golden Age of Ottoman Arts.
Con: Despite artistic flourishing under his extensive patronage, scientific surveys/institutions were inadequate. Ottoman Navy was enough to control Eastern Mediterranean, but couldn't achieve any meaningful oceanic goal in Age of Discovery. (Beylikten İmparatorluğa Osmanlı Denizciliği, written by İdris Bostan, one of a handful historians of Ottoman Naval Warfare, is a great account of how Ottomans fell behind in Naval Warfare)
Roxelana & Mustapha
Pro: /u/Chamboz is right. The bogus notion of Roxelana controlling Suleiman is a recurring theme in popular history books. There were de facto Ottoman queens in 17th century (Kösem Sultan, Turhan Hatice Sultan etc), but Roxelana never possessed that kind of power. Yes, Suleiman was in love with her (The dude was kinda/sorta monogamous. He was a poet who craved for love. After Roxelana's death, he kept seeking a woman to love). She had some sort of influence on Suleiman, but couldn't control him. And yes, Suleiman killed two of his sons: Mustapha and Bayezid. He didin't have any other choice. Mustapha was flirting with rebellion and Bayezid openly revolted.
Con: His least capable (and only living) son inherited the crown.
Conclusion: You can find wittier or bolder or more prescient rulers in Ottoman Empire, but not all in one (with the single exception of Mehmed II, who was less merciful than Suleiman though). You can make a case for him being slightly overrated, but his overall success made him a great ruler. His magnificence comes from a cumulative effect.