My most fun bike is my most expensive one for a couple reasons

1. it was pro built and tuned so everything works like a fine watch

2. Its really freekin fast and easy to accelerate, and to me fun is hammering it as fast as possible like a guided missile.

That said I often get the most enjoyment out of my middle bike (the BD Mercier I built with Campy Parts), mainly because I can ride it more often... taking a triathlon bike out on the streets here is asking to become road pizza.