Allowing for the fact that NO lock is perfect, Kryptonite makes a SERIOUS bike lock that uses both hardened chain and a hardened lock that can only be cut with a grinder. No universal key (bolt cutter) or saw will touch this. A hammer won't break the lock.
https://smile.amazon.com/gp/product/B001JPQ0HC/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
You can also register for insurance to go with the lock.
If you want more, here's more:
https://smile.amazon.com/Kryptonite...6CDNNQFJGKH&psc=1&refRID=PVA64JEE36CDNNQFJGKH
The NY Noose chain design is efficient in that it can noose around the genny handle then the remainder of the chain can encircle a tree or post.
I bought one to protect my son's $1600 electric bicycle in the city of Denver. So far so good. Buy a second chain if you need it.
My genny is "protected" with conventional 1/4" trucker's chain and padlocks. A universal key would make short work of this, but ONE MUST BE CARRYING A BOLT CUTTER or a big hammer to break the locks. 5 years and no loss so far.
Gennys don't like to get wet. I have a first line of defense, and that is to "tent" the genny with a 42 gallon lawn and leaf bag. See photos. The fiberglass driveway marker makes a good ridge pole, held in place by a couple of knob bungees. Three stakes hold three corners of the bag. A large rock holds the 4th corner to enable easy access to the start pull and controls. The tent design keeps the genny dry but allows more than adequate ventilation, and the bonus is that the bag serves as camouflage for the genny to any but the most observant eyes. (Vs. bright orange or tempting Honda red.)
P.S. I don't need much power, so I locate the genny at the far end of a 100' 12/3 AWG extension cord, so the noise is far from camp. (we boondock exclusively, so the genny is VERY far from neighboring sites).