If I could own one genuine arcade machine, I would choose Gravitar. You can keep your hydraulic cabinets and realistically rendered 3D worlds. Seriously, Gravitar represents to me the golden age of arcade gaming and when you add that to vector graphics and gravity gameplay... well, I'm in heaven.
The gameplay is what's become the staple of gravity games since: Limited fuel, delicate controls, enemies and gun emplacements to destroy and extra fuel to pick up. You start in a solar system, each planet representing a different level offering an increasing challenge based on the score available. The sun in the centre of the system will pull you inwards to destruction if you allow it. Destroying all the guns on a planet and escaping will cause the planet to explode and it's onto the next one.
Being a vector game the controls and visuals remain completely smooth and responsive. It's a lot faster paced than most gravity games, and very demanding in terms of available fuel. There's not much room for messing about! One of my favourite things about it is the way the levels scale into view and fill the screen, the gaming universe bursting into life from a single dot. On the planets themselves there's a great crash-zoom effect as you get closer to the ground. It's a simple thing but it really adds to the dynamism of the game.