When you ask for the best in paint products you will get quite a few Opinions and they are just that Opinions. There have been multiple tests involving various Epoxies and each one is usually skewed to show one out performs another. With that being said, I have used several products and each had there pros and cons. The biggest con was whether the company will stand behind their product.
The current products I use for sealing bare metal depend on where it is your sealing. Interior surfaces of a frame where it very difficult to apply epoxy will get 1 of 2 products (due to availability) are 3m Cavity wax (08852 Comes in spray can form and applied using a wand of varied lengths for different situations) and the other is a product called SEAL OUT ( haven't found this a quite some time, comes in a gallon jug and is also applied with varied length wands). These are both non hardening yellowish in color waxy type products that will creep into every crevice to prevent rusting. You will occasionally have to wipe excess from drain holes. Only apply after painting as paint will not stick to these products.
Exterior surfaces get Valspar CV50 EPOXY (Comes in varied colors but usually black for factory look) This is a very durable Epoxy and simulates the factory satin black finish very well. It becomes quite hard and scratch resistant. Choice of color can be applied directly over it if you prefer. It can also be made into various undercoat colors to aid in less chance of color variations in your top coat. (Note: they also have a high build primer surfacer CPSHS that can be ordered in various colors as well). Down side is, it isn't cheap.
I have not used the KBS systems coatings, so unfortunately I can not state good or bad with it. Appears to be less expensive and easier to get than other products.
PPG's DP line was a good product back in the day, but since they switched it to DPLF (Lead free) it is not as good as other products. Is more expensive that other products.
SPI Epoxies are ok. I don't like the finish as it sprays out though. Perhaps it was a bad batch, but I prefer my current product over this. Plus side is, Less expensive than other products.
Dupont has sytems which are just that a SYSTEM. You will need multiple products used in order or they will not stand behind the application and warranty.
The best philosophy is KISS ( Keep It Simple Systems) The more times you have to go over a product the more chances you have of something going wrong, aka, Wrinkling, Dirt, Blistering, Peeling due to non adhesion, etc.