Most greenhouse owners will want UV resistant greenhouse plastic sheeting. A plastic that also as an anti-condensate (AC) additive is also a good idea since it can get very humid inside the greenhouse. Greenhouse plastic sheets come in a variety of thicknesses. The most common greenhouse plastic is 6 mil thickness which can be rated as 6 mil 1 year (nursery plastic) or 6 mil 4 year greenhouse film which is the most common clear greenhouse plastic. You can also get 6 mil white greenhouse plastic for 55% shade for head houses and retail spaces.
Thick greenhouse plastic options may be needed for your end walls, doors, or if you need more resilience. 12 mil greenhouse plastic is good for these applications, and you will definitely want a 12 mil woven poly in black and white if you have a light deprivation greenhouse. However, for a heavy-duty greenhouse plastic to cover the roof of your greenhouse frame, you probably will not want to go any thicker than a 9 mil greenhouse plastic because thicker plastics will reduce light transmission.
If you are wondering what the worst greenhouse plastic choice is, that would be 4 mil greenhouse plastic. 4 mil is simply too thin for practical use. Sometimes it is used for temporary structures that will only be erected for a couple of months, but truly it is not a good option even for a hobby greenhouse.