strongest
HUMANITY'S GREATEST ASSET
Levi is kind of a big deal. Right off the bat, he's introduced as humanity's strongest soldier. He is said to be worth a hundred other men—typical "one-man army" stuff. Obviously, him being as strong "as an entire brigade" is not meant in a literal sense; there's nothing superhuman about him. It simply means that he's the best at what he does: wrecking titans.
Of course, it goes without saying that it's hard to kill titans when you're dead. This is another way the statement that Levi is worth a hundred other men can be taken. It really doesn't matter how good you are at killing titans if you're reckless, overconfident, or just plain stupid—that kind of person doesn't tend to last long in the Survey Corps. This is an important thing to note about Levi's character: despite the fact that he is hailed as humanity's best bet, he does not flaunt this at all. He's perfectly aware of his own capability, but he isn't cocky. Every time the Corps ventures outside the walls, it's basically guaranteed that a large number of people will die. Though he has avoided such a fate, he doesn't take it for granted. Some advice he gives to Eren shows that he doesn't underestimate titans despite his track record, because the lack of information humanity has on them is one of their greatest weaknesses. Staying alive means being quick physically and mentally: aside from being fast in general, Levi is very good at staying sharp even in life-and-death situations, and that's just as important as being a titan death machine.
It's also implied that Levi is more skilled than anyone else at utilizing the 3DMG. This isn't exactly a shocker considering the 3DMG is essential in fighting titans, but I thought I should probably mention it..