All good, badge granted.
However you can build more efficient launchers. Here you reach orbit using Hawk engines only. Those engines are good to fight gravity, but they consume a lot of fuel. The Valiant engine seems less attractive because of its lower thrust (a single Hawk pushes as strong as 3 Valiant), but it's actually more efficient.
See this build for example:
Each side has the same thrust, so this thing doesn't move. But if you look at the fuel gauges you'll notice that one of the fuel tanks has its fuel pumped out faster than the other. And in the end:
The Valiant wins. It's in this sense that the Valiant is a better engine. When you don't need a big thrust anymore the Valiant is the best.
What you have to look to know if an engine is efficient is... efficiency:
Higher is better. If you look into the big engines the Frontier is an efficient engine too.
In practice, it's best to switch to efficient engines when you're approximately half on your way to orbit. For this reason, rockets typically have a powerful engine on the first stage and an efficient engine on the second stage. In the case of a rocket with boosters, the boosters have powerful engines, and the first stage has an efficient engine.
A good rule to follow is that no Hawk engine should pass the Karman line (the atmosphere limit). If it happens, then you don't ditch them soon enough. See how your rocket reached orbit with all its Hawk engines? It means you could have been way more efficient. This part is crucial for the Voyager quest. This challenge is precisely about efficiency.