Favourite word: Indomitable.
Honourable mentions: Imperturbable and Intangible
I've actually been thinking about this question since I first saw it. I'm something of a philologist, so of course I have to answer it, but there are so many ways in which I can love a word. I can love the sound, the meaning, the spelling, the origin. I can love a word's entire etymology, or the way the way it feels when you say it aloud. I can love a word for the subtle nuances behind its actual meaning. I can love a word simply because it is dead and forgotten, or because its sound is so well suited to its meaning. I can even love a word for no other reason than that an author I love loved it before me (Cyclopean
* and wherefore
†, I'm looking at you).
I've been having a love affair with the English Language for as long as I can remember (and my childhood memories start earlier than most). Over the course of the past two decades I have fallen irredeemably in love with all sorts of words. But when it comes down the the very root of it all, when I spend a month contemplating thousands of words and why I love them, the one word which encompasses at least a little bit of every reason for which I can love a word is a heavy, open, polysyllabic, vastly useful but rarely used Latinate word with overtones of, not nonconformism‡, but something similar to it, I think the only word to describe it is ... indomitability.
‡ nonconformism itself is one of my less favourite words word. -ism (and the related -ist) is a weak-sounding suffix.