I was in Kassel, Germany in 1992 for Documenta IX and had my brain melted by the sheer volume of amazing art.  As a teenager/young-adult, this was really my first opportunity to explore the art world in any meaningful way.  That experience of wandering around the galleries and discovering works and artists...learning about conceptual, contemporary, video/film, mixed-media and conceptual art had a powerfully profound and long-lasting impact on me as a person.

Whenever I think about this time, I almostly always think back to Anish Kapoor's "Descent Into Limbo".

(source)

The viewer enters a small concrete/stucco room in which there is a large black circle in the center of the floor.  Otherworldly/surreal lighting fills the space (was it electronic? natural? something else?), but the circle is the focus of attention.  What's so special about this?  Why do I care about this circle?  Wait?  What is it?  Is it a carpet or a mat or a flat form...or wait, maybe it has depth?  Maybe it's a hole.  Why can't I see it!?  What does it want from me?  We're a little scared and nobody seems to want to get close to it.  It has no edge/lip and there certainly is no bottom.  It's perfect darkness, or darkness perfected (I can't tell which).

Anish's site has some drawings that explain it in more detail.  I sometimes wonder if it's still there, haunting the ground in Kassel.