The correct translation would be “Voyage favori” it’s already slightly shorter than your current string.
I don’t quite understand why you want it smaller, but anyway favorite de voyage is gibberish. What you mean is probably voyage favori or voyage préféré.
If your context is clear enough (i.e. the user will understand it is about a trip), just use Favori.
Depending of you context “Voyage favori” could be an adequate answer.
If you are developping an app in ITS, you should consider “Trajet favori” as an alternative.