Because of the interstellar distances within galaxies, it's said that galaxies can pass through each other with little effect on individual stars (or their planets).
But I've long been interested in what kind of fireworks might be unleashed if the supermassive black holes at the centers of a pair of galaxies were lined up so that they actually collide...
Doesn't strike me as a nice neighborhood to be near... '-)
That could be a big kaboom, assuming black holes actually exist of course. :^) The overall mutual attraction would disrupt all the stars in each galaxy, disrupt planetary systems, etc, but the degree to which they'd be disrupted would vary based of course on the mass and proximity.