What is your favourite opening and why?

Sicilian Najdorf, with either color. Because tactics.
A Heroic Defense in the Sicilian Najdorf - Kids, don't try this at home! - Chess Forums - Chess.com

The Tarrasch Defense against 1 d4 because black gets breathing room from the get go and black's king rarely if ever comes under attack. Black does have to take on having an IQP.

French Defense with either color because it came to me naturally. In 1995, before I knew what an opening was, I was playing the French Defense.

I play my own variation of the London where I keep all pieces on the board for positional play. And it works - engine verified. It combines Catalan-like strategies with the London middlegame tactics and because of positional play, usually an intense endgame with both sides' king safety heavily compromised.
The Caro-Kann, it offers exposure to a wide range of pawn structures but the choice of pawn structure is committed early on, thereby offering a wide-range of plans with each plan being relatively easy to understand and execute. The negative side is less exposure to central tensions.
Another plus of the Caro is that, depending on the choice of lines by Black, it can be played with either a dynamic equality or a more classical equality mindset.
I'll start off with Queen's Gambit for white and the King's Indian Defence for Black.
For QG accepted, I love how white almost fully takes control of the centre from the beginning. For KID I love how the King's safety is paramount, and this can be achieved in just a few moves in the beginning.