Which components knocked you out on first listen?

Polk SDA 2B speakers first real high end audition mesmerized
Rowland model 1 60X2 But 60 amps current output=bass slam
Mirage M1 and its successor M1 Si
XLO type 5A speaker cables
Conrad Johnson premier 8 amplifiers
Dunlavy V
Wadia 27
Audio Aero Capitol both versions
Genesis 201 speakers

The answer to this question would go back to my first exposure to a "high end" system.

Vandersteen 2C speakers
ARC M100 amps
ARC SP-10Mk2 pre-amp
Sota Star Sapphire tt with SME Mk 5 arm and Koetsu cartridge
Recording Earl Wild and Boston Pops "Rhapsody in Blue"

I never knew music could ever sound like that. The first great experience is generally the most memorable. I'll always remember it because it demonstrated to me what was possible to achieve through reproduced audio.

Most recent component in my system over all others that just blew me away is the Berning zh270 quasi OTL amp. After almost a year it still knocks me out.

Beyond the above, I have been impressed by systems more than any individual component bearing in mind that I haven't had the opportunity to have a Rockport Sirius, or Burmeister DAC and transport in the system to compare.

My first "knockout" experience was when I first went to the Linn/Naim dealer that I eventually went to work for. I listened to a Linn LP12/Vallhalla/Ittok/Asak into a Naim32/SNAPS/NAXO/ and 3 NAP 250's driving Linn Isobarik PMS speakers. This system really knocked my socks off in 1982. Really opened my eyes to high-end stuff.

More recently, all the stuff I own now has really knocked my socks off. Agree with Tubegroover that the Berning amps are absolutlely killer. The Teres 245 turntable is a real "shocker" at how good that is. OL Silver Tonearm is way better than it has any right to be for under $800. The same for the Denon DL103R. The Lowther drivers continue to be generally unknown/ignored by large segments of the audiophile world, but provide stunning sound at low power input.

I haven't listened to the more esoteric stuff, just haven't lived in an area that sells it, but in my experience based on my local dealers (Washington DC area), these pieces absolutely floored me on first listen:
Ayre V-3 amp
Goldmund Mimesis 29 amp
ARC VT100 MkIII amp
Van den Hul MC-One cartridge on a full-blown Linn LP12 (circa 1991)
Linn Linto phono stage
Linn 5140 speakers (circa 1998)
ATC Active 50 speakers (circa 1993)
B&W Matrix 801 and 805 speakers (circa 1991)
Spendor BC3 speakers w/British Fidelity A3 amp (circa 1987)
Genesis 500 speakers
Vandersteen 5's
B&O Beogram 3000 turntable (circa 1975)
B&O linear tracking turntable (4004?) (circa 1978)
CAL Tempest II CD player (circa 1991)
Linn CD12 CD player
Transparent Ultra and Reference line of cables (circa 1994)Ensemble line of cables and power conditioning stuff
full on Meridian home theater/music system (circa 2000).

Also want to add to my list two components I am currently using together--agree w/TOK 20000 about the ARC CA-50:
ARC CA-50 integrated paired w/Proac Response 1SC's: I love this sound as much as that of my big "reference" system (Genesis 500's w/ARC VT100 MkIII and BAT VK30SE ). It absolutely floored me when I first hooked it up! And both pieces bought used here on Audiogon, in perfect shape.