Most satisfying system change in 2017?

Synergistic Research Black Fuses.  Believe it.  Those just amazed me when all I was expecting was maybe a mild bump. 

Had several changes this year. The biggest just happened as Bob Smith at TDSS put the Level 3 mods into my Nuforce Ref 9 V3 SE mono amps about a month ago. The sound continues to improve as the power supply and wires break in more. It is more resolving, dynamic, and even more quiet with stronger, better defined bass.

Added 5 SR Blue fuses to my amps, MW 5400 CD, and Whest 3.0 RDT SE phono pre--made the sound much more beautiful, fuller, more dynamic and detailed.

My VMPS RM40 BCSE speakers have never sounded better.

Going with all Cerious Technologies Graphene Matrix cables was as big as any component Change.

asp307,
The Chord Dave had the most impact on my system, but it was only one of several changes and they occurred at different times. I changed from Wireworld Platinum XLRs to Audioquest Fire XLRs, from Wireworld  Silver Eclipse 7 speaker cables to Clarus Crimson speaker cables, and most recently changed the standard spikes on my speakers to IsoAcoustics GAIA Is. 
 

Made a lot of changes this year...
Misc:
Installed 51 feet with ceramic balls in them 
Installed 5 Oyaide Tunami Power cords 

Digital:
Replaced an AQ cinnamon digital cable for VH Audio silver 
Replaced a Parasound Zdac for Audio Alchemy's latest
Replaced a Pioneer stable platter with a Cambridge transport

Analog:
Replaced a cardas phono cable with a furutech
Switched out a Denon DLs1 cartridge for a Clearaudio concept MC
Installed a VPI Analog Drive System

Removed all vacuum tubes from my system:
Replaced a Sonic Frontiers Phono amp with Audio Alchemy's latest
Replaced a 6550 SET Jeff Korneff power amp with Valvet mono blocks
Replaced a melos preamp with the Audio Alchemy preamp/dac 

All of the above changes improved the system dramatically with the single largest improvement being the SE solid state Valvet mono blocks. These are sweet amps!