BEST UPGRADE regardless of cost?

A reel to reel tape player.

;-)

For myself there may be two I'd consider best. First being the acoustic treatment/taming the listening room and second a change of speaker cable.
This may be strange but I removed a Transparent Ultra from the system and replaced it with a vintage Tara labs and never looked back. Whoever bought the Ultra here on agon got it for pennies and I hope it works for them, it did nothing for me.

@khrys. I think your blanket statement on other AC power products is incorrect.   You might enjoy learning about balanced power from Equitech. Maybe :)

Spectral Pre and amp with MIT cabling. Spectacular improvement over the other “so called” top level equipment I had. I’m forever greatful to the well known,well respected Michael Percy (Audio) for his knowledgeable guidance. (PS for those unaware; both Spectral and MIT have extraordinary engineering talent whose expertise is respected by those in other fields :). 

Acoustic treatment