If something is splashiest, it means it has the most splashes or is the most showy and attention-grabbing.
Splashiest things are often bright, colorful, and exciting.
Examples:
The splashiest part of the parade was the big float with all the lights.
Her dress was the splashiest one at the party.
The splashiest fireworks were the ones that made shapes in the sky.
The splashiest fish in the tank was the bright orange one.
The splashiest car in the race was painted with all the colors of the rainbow.
The splashiest thing in the room was the big, shiny disco ball.