Leo is one of the most popular robots, and for good reason. Renowned all over the WR playerbase, wiki and forums for its heavy armour and firepower, it is capable of taking on any role and performing well at it. Furthermore, it only costs 1.7 million silver, and has 226,000 health at level 12 mk1, holding the record for the robot with the highest health until finally surpassed by the Raijin with 250,000 health at the same level.
Firstly, we have its durability. With 226,000 hitpoints at level 12 mk1, it has enough armour to literally take on any opponent, and survive the encounter. For many, it also provided an extra layer of armour to protect it from snipers and mid-rangers, with enough remaining health for brawling. The Leo also has an average profile for a heavy robot, and isn't necessarily a large target, especially when compared to the Butch, Fury and Natasha. Even the Griffin sometimes is a larger target than the sleek hull of the Leo.
Secondly, the Leo is extremely versatile. With one heavy and three light weapon slots, it is able to remain effective in any role. For close range fights, it can be equipped with a Thunder and Magnums, Punishers, Gusts, Aphids and Pinatas, providing excellent firepower on the battlefield, dealing high damage. For mid-range engagements, a Zeus, Trident or Dragoon with Gekkos and Pins provides decent support fire, and finally a Trebuchet with Gekkos or Arbalests and a Zenit with 3 Noricums for long-range. In the low-mid leagues, especially in silver, the Leo is the most common robot, almost ubiquitous, mainly due to its durability and versatality. While it is slightly outgunned by a few of its rivals, such as the Natasha in sniping, its high durability often means that it will come out of the battle victorious.
Finally, the Leo gives its pilot excellent firepower with one heavy and three light slots. While its firepower isn't much more than its rivals, sometimes even equal to or less than, the tactical flexibility provided by four slots mean that it is able to sustain critical hits better than robots with less slots, such as the Raijin, can while still dealing lots of damage. It is best suited to close-range brawls where it can make the most of its firepower and armour, but it is also capable of many other roles such as sniping.
Overall, due to its incredible durability, firepower and versatality, Leos continue to be seen relatively often in the higher leagues, and although they are eventually outclassed, they continue to be an intimidating and powerful presence on the battlefield.