Sesquipedalian, a big word for big words.
Soliloquy is my favorite word. I get to use it slightly more often now than I used to, for obvious reasons.
Today I used the word nexus during a job interview.
I love “haecceity” - both the sound of it, and its meaning. All those vowels – it’s like silk. (Firefox’s spellchecker does not like it, though.)
Also: steatopygian. No alas or alack there, though.
The funny thing is - I’m German, and I like ésprit de l’escalier much more. I understood what it means only when I heard the French for the first time, too. I had thought Treppenwitz meant just a bad joke.
I’ve had the word ‘quincunx’ stuck in my head for the last few hours. It’s a great word, though it does sound kind of vulgar.
It’s not so much the word itself that the concept I want to grasp, firmly.
I really like “poliorcetics”. Means the art of siege warfare. I love that that has its own name for some reason.
[QUOTE=garygnu]
“Penultimate”
I try to use it whenever I can, but how often does second-to-last need to be labeled?
[/QUOTE]
I like ante-penultimate even more. Because sometime a guy’s got to say “the one before the one before the last one” without sounding like a complete muppet.
There’s one French word I love, not because of its sound or meaning but its etymology: vasistas. It’s a type of window built into or over a door (I think it’s called a transom in English), and it got its own name because whenever Germans visited France and saw these kinds of windows they always asked people “Was ist das ?” (What’s that ?). So now das ist ein wasistdas 
That actually is a wonderful concept, and a useful word/phrase that is missing from the English vocabulary.
Deleterious
*Sinecure. *A job with little or no actual work.
Crepuscular.
A brazilion is, obviously, the population of Rio de Janeiro in a SF dystopia – one way or the other! 
One of my favorite things is doing “legitimate” coinages – words that “follow the rules” for combining Latin or Greek roots but which for varying reasons have never been put into use. For example:
Monocerotivore – a hypothetical predator whose diet is strictly unicorns
Erythrorhyncheous – like W.C. Fields or Rudolph, possessing a red npse
Heptapodal – having seven feet
Coin some of your own!
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Here are a couple of real words that fit the thread specs:
Ultramafic – This is a real term in geology, referring to a group of igneous rocks classed together by their mineral composition, but isn’t it deliciously evocative?
Ithyphallic – Technical term in art and anthropology scholarship for figures with a disproportionately large penis. “Hentai is inherently ithyphallic.” 
Thank you for opening the door to two-word examples, because I have a new favorite one: batgirl quicksand.
“kerfuffle” is a favorite of mine. I rarely get to use it.
The word that would not die.
Is it snark at this point, or has it descended into a running in-joke? This is the third thread that I’ve found it in. Second, if you don’t count the original. (Fourth if you count the poll.)
And I’ve never found the opportunity to use the word coprophagus, which the Urban Dictionary claims is a Zombie of little brain. I found that out when I checked on the spelling. Also that there is a band going by that name. Ah, kids today.
If you combine it with ‘rictus’, you have a shit-eating grin. Which reminds me of a line in Gaiman’s American Gods. “He grinned like a fox eating shit off a barbed wire fence.” If that isn’t an old folk usage, it ought to be.
Tomfoolery, kerfuffle, copacetic have all been mentioned.
Subsume is a new favorite of mine.
Epiphany, not the religious context, is one of my personal favorites.
Lollygaggying.
I missed my chance at Necronomi-con last year when the theme was “phobias”, and your nametag had some phobias you could check off if you “had” them. One was coulrophobia, fear of clowns. Another was somniphobia, fear of sleep.
Too late I figured out the perfect joke. I could have pencilled in “Somnicoulrophagophobia”, which is close to “can’t sleep, clowns will eat me”.