My tube amp, because it appears to be alive. Otherwise my modest (priced...bought stuff cheap or used here and there) system doesn't seem to lack anything it might need for musical enjoyment at this point, so I'll change things only out of boredom, curiosity (would a different TT sound better?...hmm), esthetics (I like striped ebony...I want some speakers with striped ebony veneer), failure (a component, not my personal failure, although deafness or sudden death could have an impact and shall be avoided if possible), or everything (except me) falling down a sinkhole...hate those sinkholes.
Of all your gear, what piece impresses you most?
I like all my gear, yet the single piece that blows me away the most is my phono preamp.