The most fun I've had flying remains to this day towing glider in a C172. In the intervening years I've flown as an instructor on piston and turbine singles and twins, flown small and large biz jets, and I've also flown every Boeing from 737-777 excluding the grand lady herself the 747. Given the choice, I'd be back in the C172 or C208 doing what I found to be the most fun aviating. Airline flying these days is a job, where I am it's as much about managing fatigue, balancing a work life balance and not letting the negative thoughts outweigh the positive ones. Is it a fun job? sometimes....mostly it's just a job.