The longest lasting and quietest engines will be the overhead valve models with a silencer muffler. Honda, Kawasaki, Yamaha, Subaru, Kohler, Onan, B&S, Tecumsah, Generac, etc. all make good overhead units.
MHO here:
The BEST is the Onan or Kohler 1800 RPM twins cylinder units. Heavy and expensive, but they are quiet and extremely reliable.
The next best is a Honda powered unit due to the variety of models, reliability of the engine and silencer muffler.
I rarely use a generator so I have the third option. A flat head Tecumsah powered Generac 5kW unit. It will run a gas furnace, freezer, fridge, lights, TV, and microwave with power to spare. It also loudly tells the neighbors I got power while they shiver in candle light! It is 1/3 the cost of a comparable Honda powered unit and 1/2 the cost of a Generac with an overhead valve engine
Unless you are contractor with cycling usage, do not get an idle back option. If the generator is running you will usually be drawing enough current to make the idle circuit useless. Gauges are nice to balance your loads.
-John