Which whole house generators put out a good sine wave?
As others have said, this will vary considerably from one device to another.

One way to measure the quality of electricity is "Total Harmonic Distortion" (THD), which is a measure of how much distortion from a true sine wave there is. A perfect sine wave is 0%, and a square wave (very poor) is 48%. Anything under 5% is generally considered "perfect" for household usage, and a good generator should be around that. But a cheaper generator could be 20-30% +. You may have to do some digging to find the specs for a model since not all manufacturers advertise it prominently.

Frankly, it's hard to tell whether your home devices will be troubled by high THD or not. It's not nearly as simple as "electronics"... most electric devices today have a DC converter to supply the device with low-voltage DC power, and actually are pretty isolated from the mains power. I think most DC power adapters are pretty tolerant of poor power, wide voltage ranges, wide frequency ranges, etc.

Note that there are other factors that may vary, like AC frequency (in the US that should be 60 Hz with very little variation), voltage (in the US should be between 110V and 130V for single-phase or 220V-260V for split-phase).