The best combat folder?
Its easy Kalashnikow assault rifle with folding stock! The make and model is pretty subjective matter, I personally would prefer Russian made AK-74 in 5,45x39 mm for regular combat and AKSU-74 (the shortened version, SMG size) for close combat situations. Probably backed up with Glock 19 in 9 mm, but it doesnt fold...
Sorry Folks, couldnt resist.
If seriously dont mess terminology, please! Messing terminology more than likely you couldnt distinguish your right hand from your left one at pretty short time. Saying combat folder in knife context you should have in your mind the folding knife what is specially designed for blade to blade dueling, dedicated for this purposes or at least for hand-to-hand combat. So the answer is pretty easy and natural no one folding knife is any good for these applications. All you could hear about such knives from makers, manufacturers or another knife users is merely feeding a line.
Even exaggerated medieval Spain navajas with 10-inched blades plus 12-15-inched handles were only a poor substitute for real swords called more to threaten showing that you are armed than for fight, especially against real sword. There are no combat folders at all.
If you are going to use your knife for self-defense applications occasionally, if situation forces, each strong, heavy-duty folder with lockable blade and handle what provides secure grip would be equally good, or equally bad, depends on point of view. There are plenty of such knifes around, usually they are called tactical folders what is also not too accurate name but it is common. Such knives are primarily the tools as each other knife, especially folding one. Calling them combat you are messing their primarily purpose. Of course this no way should mean that occasionally they couldnt be used as self-defense tools...
Please remember first was the Word, so dont deprive it of sense!