Before you decide on a brand which for most would be a Generac, look at the warranty specifically for the clause that negates it.
Most do not understand the ratings which are not normally spelled out. The highest and most reliable are units rated for prime use. That means the unit can be run constantly 24/7 until an outage for maintenance is needed. For smaller units that would be an oil change. You cannot run a Generac like that without voiding the warranty.
You’ll pay a lot more for a prime unit compared to one rated for standby use only. Some prime units for consumer use can be run for almost three months before requiring maintenance. With an option that can be doubled.
Recognize a diesel whether fueled with oil or gas, either natural or propane, will last longer than a unit designed for gasoline. At least one vendor’s offering has been known to run 40,000 hours before needing to be rebuilt. I did not add too many zeros to that figure.
Prime units will always run at 1,800 rpm.
If they are a four-pole alternator ~1800 is about right. Have seen Caterpillar gensets turning ~1200rpm with a 6-pole setup.
A pair of 140kw G.E. driven by 8-71 Detroit at 1800rpm went past 40,000 with regular oil and fuel servicing as only engine maintenance.
This also would be a contender for longevity: