This one is easy. Not a modern game by any stretch, but one game that haven't played in ages but have some great memories of is arcade classic, Kung Fu Master.
Released in 1984, when I was just 13 years old, I first encountered the machine at our local cafe. There was always a crowd of Older Boys hogging the machine, and all I got to do was stand and watch (right next to the player if I was very lucky!) Although I never got to play at this time, I learnt all the tricks. When to punch, kick and flying kick to get the highest possible score on each level and all the tricks in tackling the bosses on each level. How to take down the knife throwers, and the flying little people, the dropping vases and snakes they spawn.
If certain enemies got close enough they grabbed you, draining your energy and you had to wrench the joystick back and forth quickly to shake free. Those techniques are still in my head to this day including the music that to me is as iconic as the Mario Bros tune is to most gamers.
It was a year or so later when the local fair ground that always came around at Easter and through the Summer holidays, had a few of the machines (along with the first of the 4-player Gauntlet machines which remember you could never get close to!)
I was hooked. Using all the skills I'd learnt from watching the Big Lads, I burned through that game, pumping in 10p after 10p until I reached and eventually beat that final 5th Boss. Then, of course, the levels start again only MUCH harder.
Since those days, I have played an emulated version, but it is nothing like the feelings I remember playing the original machine, feet sticking to the floor from dropped candy floss and the blasting 80's pop music in the back ground, friends shouting support and commiserating losses.
Seeing this speed run where the five levels are completed in 6 minutes puts me to shame.