Purchased new the Toyota FJ Cruiser comes with a sheet metal engine protection plate and possibly the factory FJ Cruiser IFS Skid. The sheet metal plate is intended to only protect the underside of the engine from road debris. The factory skid will protect the forward part of the engine and some of the IFS components. I consider the Toyota Skid as more of a front bash plate that is not very durable.
The best skids are tucked tight under the FJ protecting the key elements while still proving the best clearance. Selecting a system that provides a smooth underside will help if you need to slide the FJ along a rock. I have All Pro skids installed on my FJ and they hug the underside very well. The only problem in the transmission skid is 2” higher than the transfer case and IFS skid which can hang-up on a rock. (I have had this happen)
Bud Built, Rasta and Man-a-fre offer very good skids that provide a smooth surface on the underbelly. They also provide a fuel tank skid. I don’t worry about the fuel, it is made out of high density plastic and is very hard to damage. However, if you are planning on very hard core off road the fuel tank skid would be a plus.