Your favourite word untranslatable into Standart/English.
Alas, a difficult question. Languages can be beautiful the Galaxy over, and of course they can be beautiful simply because of the sentiment someone is trying to convey.
I think that my favourite would have to be a Romulan word. Mnhei'sahe - it literally translates to the Ruling Passion and is often rendered by the Universal Translator as honour. Such a banal description is hardly accurate. Mnhei'sahe is more than honour, it more than duty, it's also spirituality and patriotism. It is the weary balance between a person's personal duty to themselves and their family, their duty and sense of patriotism and of course their spiritual duty as well. It's often an intricate and complex concept - one that, in navigating, has lead to the deaths of many wayward souls that misstep on its path. Mnhei'sahe is so complex because everyone's duty is different and everyone's priorities are different.
If you take my life as an example - yes, I betrayed Starfleet, but I did so in an act to protect the Maquis. On this occasion, my duty to protect the civilians of the DMZ outweighed my duty to Starfleet. My honour would not permit to see those people suffer due to the ill-advised actions of the Federation Council. Of course, Captain Sisko would argue that there's no higher calling than to the uniform, but sometimes there is the duty, not only to one's conscience, but to mercy as well. I may have sacrificed my career and therefore my honour as a Starfleet Officer, but I maintained my duty to Rebecca and the other colonists.
Apologies for such a long answer. But I think that you can agree such a beautiful and unique concept needs a more lengthy answer.