With the random experience I have(almost everything other than dozer, scraper, logging and mining grade), I have to say excavator is the most comfortable with entertainment, and backhoe is the most skilled/entertaining. Front loader and blade are probably the most boring, up there with rollers and forklifts. Small stuff requires a lot of up and down, or cramped legs. Skidsteer on a patch crew requires the most technical ability.
I'm not even an old guy yet, and I'm leaning towards the machines that can be ran smoothly(excavators and backhoes). So if they move fast and doesn't have a boom and stick, they are usually rough on your back and tail end.
I'm finding a lot of operators don't understand limitations and quirks of machines, and will try to use them as a battering ram rather than a hydraulic tool. My favorite part of the job is proving that you are more efficient if you understand the machine and the leverage and physics involved, and don't just beat the machines up.