This is tough!
It's not 'in game' necessarily and I'm not much of a pins guy so I never got the pleasure of hearing it nearly enough, but the old match-knocker slamming at the pinball machine is music to my ears. There's something really, really satisfying about the DK - Needley NEE nee too.
Of course, all the gauntlet voices. I love the SF2 credit/start sounds (usually my text notifications!) but the two end-all-be-alls for me in two different X-Men games for some reason...
Colossus - RRAAAAARRRRGGGGHHHHH (we all knew one kid that would just plop tokens into the machine simply to spam that yell!)
and
Wolverine - BERSERKER BARRAGE
And not arcade, but the ka-CHING-pssshhh of Duck Hunt and Hogan's Alley on the NES is forever etched in my brain.