Thanks for the fun topic! The very first "WOW", and the one that really cemented my early interest in this hobby was a binocular view of the Arend-Roland great comet. My sixth grade teacher came back to the school after sunset and brought his binoculars to show several of us the comet. This comet was easily naked eye and was notable for it's bright, sharply distinct anti-tail (at the time, no one knew why this comet had an anti-tail or what it was).
Back in the days when I only had a poor quality, home-built 6" scope, a hugely significant WOW for me was getting to use John Dobson's 18-inch scope all by myself for the last 3 hours before dawn.
Not too long after that, I built a 17.5" Dob that, to this day, continues to bring one WOW after another.....
--- Seeing the blue and rose colors in M20, the Trifid Nebula for the first time
--- At 555x, easily seeing the ansae with their little knobs on the ends poking out from the sides of NGC7009, the Saturn Nebula
--- Faint challenge objects can be WOWs too...like seeing my first globular cluster within M31, the Andromeda Galaxy (I've since observed well over 60 additional globulars within M31)
--- My first time ever (after many tries in various scopes) seeing the Horsehead Nebula. Using an H-Beta filter and 222x, I could actually see the indentation in the bridge of the horse's nose
--- Vega at 150x or 222x, a blue-white diamond in the sky, simply stunning!
--- Can't forget to include the globular cluster M13 in this list, awesome at 222x in the 17.5"
--- Likewise, can't forget to include M42, the Orion Nebula, words simply aren't enough
--- Lunar grazes of Regulus and Aldebaran and occultations of Saturn, Jupiter, Venus and the Pleiades
--- Barnard 86, the "hole in the sky". Stunning in large scopes, this dark nebula sits right next to a nice tight open cluster just a few degrees south of the Lagoon Nebula
--- Seeing what I thought at the time were bow shock waves in front of comet Hale-Bopp's nucleus (turns out it was a spiral of dust coming out of the nucleus!)
--- All the amazing detail visible in M17, the Swan Nebula, with a UHC filter at 222x
--- Viewing the globular cluster M22 at high magnification in exceptional seeing conditions and actually resolving the Milky Way background around M22...the entire field of view was stars and more stars
--- The first time I ever saw the very hard-to-see pulsar in the heart of M1, the Crab Nebula
--- Seeing the spike portion of the Veil Nebula look 3-dimensionally tubular with an O-III filter at 222x
--- My first time seeing filaments in M1, the Crab Nebula using extreme magnification and (I think) an O-III filter
--- Clearly seeing Enke's Division in Saturn's rings (not to be confused with Cassini's Division)
--- Omega Centauri, whether you're viewing with a 4" APO or a large Dob, this globular cluster resolves easily and is beyond amazing, even when viewed at only 5 to 8-degrees elevation
--- And galaxies, like gorgeous edge-on NGC4565 with its dust lane, the spirals of The Whirlpool M51, The Blackeye M64, the HII regions of M33, and so many more
--- Jupiter, Jupiter, Jupiter, amazing planet, like a mini-solar-system, always something happening