Even more than he likes to drink, Tony Stark likes to tinker. From the first version of the Iron Man armor made of literal iron in 1963, to his current version (an Iron Man suit made entirely of energy), there have been more than sixty iterations and evolutions in Marvel's main universe alone. However, the coolest by far remains Iron Man Armor Model 51... also known as the Model-Prime.
In the larger history of Marvel Comics, Tony was inspired to make the armor after 15-year old Riri Williams a.k.a. Ironheart managed to create her own version of his older armor from scratch. To prove his abilities, Tony locked himself in his lab for weeks until he had come up with something new. And sure, the resulting armor wasn't flexible or powerful enough to survive his fight with Captain Marvel in Civil War II. But even if the suit wound up shelved after Tony fell into a coma, there is no denying the Model-Prime's greatness among true fans.
To understand the Model-Prime Armor, fans first need to understand two very specific suits that came before it. The Modular Armor (Iron Man Armor Model 13) could swap out different modular components for different tasks, rather than changing the whole suit. For instance, if Tony needed a gun arm, he could swap out his gauntlet for one (most noticeably, it also featured some of the famous Iron Man 'Hulkbuster' attachments).
Using the Extremis Virus as an inspiration, Tony Stark created the Bleeding Edge Armor (Iron Man Armor Model 37). Housed entirely in his body and controlled by his thoughts, the Bleeding Edge Armor could create any weapon or tool Tony needed using nanotechnology. Tony's suit in the movie Avengers: Infinity War is an homage to this, as audiences witnessed Tony create guns, shields, missiles, and even medical devices out of its various nanobots.
The difference between the movie version and the comic book version is that Tony could not take this suit off; Bleeding Edge was literally a part of his body. While at first this seems like benefit--Tony was never without the armor's protection--it also proved to be the armor's major weakness. Model 37 could not be operated by or protect anyone other than Tony Stark; (unlike say, the Mark 42 version in Iron Man 3).
Model-Prime combines the strengths of both these armors without their weaknesses. Comprised of interlocking hexagonal scales, the armor could be shifted into nearly any configuration, giving it Swiss-army-knife versatility without resorting to altering the user's biology. The armor could be sized for anything from Hulkbuster version to a child. It could also be piloted remotely, unlike Bleeding Edge. For that 24/7 protection, the armor has an invisibility setting so Tony could wear it while appearing unprotected. It also could be contained in a small wrist device. Bonus fact: Later editions were even immune to Magneto's powers, using carbon instead of metal.
Nevertheless, the Model-Prime Armor remains Stark's best suit by far. First of all, it just looks cool. The scale-like plates and triangle-shaped face-plate make it look sleek and reptilian. Dr. Doom even adopted his own version of the armor for a time, and that villain has nothing if not style. Most importantly, by combining the functionality of all his previous suits into one, Tony Stark proved that why he really is the one and only Iron Man.