Diesel
The job-site default: high output, tolerant of long runs at load, large base tanks. Downsides are noise, emissions and fuel logistics, plus problems if run at very light load for extended periods.
Natural gas
No refuelling where a pipeline connection exists, cleaner emissions, but fixed in place, slower to accept large step loads and dependent on connection work and permits.
Propane
Stores indefinitely, starts well in cold weather, and burns cleanly. Output is typically slightly lower than the same unit on gasoline, and tank supply has to keep up with demand in cold conditions.
Dual fuel and hybrids
Dual-fuel portables switch between gasoline and propane, which is genuinely useful during an outage when one fuel becomes hard to obtain. Battery-plus-diesel hybrids cut engine hours and noise on longer jobs.