There are so many gorgeous arcade games out there with all sorts of different visual styles that it's practically impossible to pick a favourite, but gun to my head I'd probably say 2D sprite art (with a focus on detailed animation) is the most timeless to me.
Either Street Fighter 3: Third Strike or Garou: Mark of the Wolves would be the best examples of this. To this day I think those are some of the best looking games ever made.
Metal Slug is right up there as well though!
I'm also really fond of the visuals in Cave shmups - Mushihimesama, Ibara Kuro and ESP Ra.De. especially.