There are a few different types of sealant that can be used on a transmission pan. The best type of sealant to use depends on the material the pan is made out of and the environment it will be used in.
For example, if the transmission pan is made out of aluminum, then an anaerobic sealant would be the best option. This type of sealant cures in the absence of air and forms a strong bond with metal surfaces. However, it can take 24 hours or more for the sealant to fully cure, so it's not ideal for applications where a quick fix is needed.
If the transmission pan is made out of cast iron or steel, then a silicone-based sealant would be a better option. Silicone-based sealants cure quickly and can withstand high temperatures, making them ideal for use in transmissions.