I can't say for sure which one is my favourite. I do, however, have several dungeons that I adore for various reasons.
In no particular order.
1) Braflox's Longstop
This was the first real test dungeon when I was leveling up around release day.
The environments are gorgeous, enemies are fun, and it just had an excellent vibe.
The boss was the first time I genuinely felt challenged. This was when everyone was new and still learning.
We wiped for around 30-35 minutes on Aitar, and I loved every minute of it.
2) The Sunken Temple of Qarn
Mainly for the boss.
This was the first dungeon that I didn't complete my first time.
We wiped for around 20 minutes on the first boss, and probably 40 on the 2nd boss.
Didn't finish the dungeon, 90 minutes gone, and it was totally worth it.
3) Hullbreaker Isle
Environments. Enough said.