Best debut album

Dire Straits  

Oh, just one? That’s easy: Music From Big Pink. It changed the world for every good musician I know, and many I don't (Eric Clapton, Nick Lowe, Neil Young, hundreds of others).

Spirit
Dan Fogelberg

I am going to go a slightly different route from my learned colleagues here and take the path slightly more obscurious.

The Least We Can Do Is Wave To Each Other .... Van Der Graaf Generator.

A Blueprint of the World ..... Enchant.

Mirage ..... Camel.

Motörhead S/T.

All truly superb first efforts.
And paved the way for even greater successes.

And if you want a favorite out of those it would have to be Van Der Graaf

petg60 -- Pink Floyd's first album was Piper at the Gates of Dawn.  I actually went looking for it in my vinyl collection.  Damn, I must have sold it eons ago.  I bought it for Interstellar Overdrive.