If you are driving on Route 40 to Eilat, you will go through Makhtesh Ramon, the largest and most well-known of makhteshim. In Mitzpe Ramon on its northern edge there is a visitor center and a chain of viewpoints over the geological formation. Viewpoints are all free to access, including that right by the visitor center. It is quite easy to take a look at the makhtesh with a reasonably short stop on you driving journey, which is what I did in November of 2019. I did not go inside the visitor center at all, and have no first-hand knowledge of what it might offer for a park-visiting fee. (Note that Israel Pass includes Mitzpe Ramon Visitor Center in its list.)

Makhtesh Ramon at first looks immense and sufficiently awe-inspiring, but after a few minutes of standing near the edge that impression started to dissipate. It is a large closed valley with steep walls, a landform that is apparently native only to the Negev desert and the Sinai peninsula. Maybe the relative uniqueness boosts its OUV, but I'll be surprised if it ever makes the inscribed list. Admittedly, I do not have a good gauge to evaluate OUV for purely natural monuments, so your mileage may vary.