It could be visually beautiful, mechanically or otherwise.

I’m a visual person, so I have to go with Photosynthesis. It has an amazing table presence and the artwork is amazing. Seeing the trees slowly fill the clearing and, if the lighting is right, cast their shadows is very special.

What's your most beautiful game?

@tabletopgamesblog I think Everdell is the obvious choice here (excluding the debatable tree). I also love Root in terms of design and mechanics. Love Ian O'Toole's board designs for their clarity and overall cleanness - Irish Gauge, Clinic Deluxe (waiting for Weather Machine by Lacerda). I would add Oath by Leder Games to the list, which is beautiful.
@qubus Ian's graphic design certainly helps the aesthetics of a game.