Mine is ‘canys’. It’s an old word used often in the bible. Though I’m not religious, I do love how beautiful it sounds and how subtle it is. It means ‘because’ or ‘since’ and is used before an adjective.
But what about ‘pendwmpian’ or ‘slebog’…?
https://nation.cymru/culture/best-words-welsh-language/
What is yours?
A phrase more than a word - “mae hynny yn debynnu” = “that depends”. I try to make a point of saying it at least once a week. 
Sbigoglys!
or pendramwnwgl 
Saying and feeling “Bendigedig!” is a great joy. 
Datblygu… Have had lots of it here over the years!
My favorite word changes as I learn new vocabulary but Bendigedig is great fun to say! I guess some of my children like it too because they named the neighbors new horse “Bendigedig”.
my almost 3yr old has a new favorite word as well…
David: Mommy can David have a cheese stick? (speaks in third person… the joys of language learning)
Me: sure
David: Coffi Du!!!
I thought maybe it was a funny fluke but then yesterday we went out to eat and when I brought him his food he shouted “Coffi Du!!”. Apparently in our home the words “coffee du” mean HOORAY
That’s a super cute phrase.
It’s fun living next door to the village of Pontrhydfendigaid = The Bridge over the Blessed Ford. The “Blessed” refers to Abaty Ystrad Fflur = Strata Florida Abbey on its outskirts.
Great story about David’s vocab.
I actually really enjoy eisiau - I think it looks and sounds lovely, and shows up so often.
I also love seren (star)
I’m undecided, but ailwampio is a very strong contender.
Can I throw in a vote for dychmygu? 
Gwiwer, squirrel, obviously.
I’ve loved it ever since Aran introduced this sentence into the notoriously difficult Challenge 14, Level 1.
Mae’r wiwer werdd yn wiwo mewn wigwamau wyffurf.
Not exactly an everyday kind of sentence 
So many. Cwtch, because it was my first word. Smwddio because when, I don’t know.
I’m not fond of dallt if it has the soft mutation. My mouth really struggles with it.
I like ‘llall’ - 5 letters, four of them l’s, and a satisfyingly spitty sound that really lets you get your teeth into pronouncing it.
Pedant alert!
It’s only three letters. Ll is one letter in Welsh. Because of this you will find that llall comes after lwc in the dictionary.
But I agree that it is satisfying to spit it out.
For some unknown reason, I like dychrynllyd
Sgwennu. Just sounds lovely to me.
I like smwddio partly because I’m pretty sure you can also use it for, eg, stroking a dog - where it’s entirely appropriate, but you’re definitely not ironing.
Slwtshlyd. Meaning squelchy. The word actually sounds squelchy!