Off-the-shelf stuff would be better than nothing, but not all that great. Google knife-proof vests to see what the trade-offs are between ballistic vests and something that will stop a blade. Given access to modern materials and fabrication techniques…you’d end up with something that looks a lot like a modern-designed suit of medieval armor, with some limited use of materials they didn’t have to make it somewhat lighter for the same level of protection. They had weapons technology pretty well figured out.
Modern fabrication and materials science would improve the average quality, and of course we’d completely out-compete them on how fast we could turn it out if we had to mass-produce it (assuming modern factories doing the work) but really, steel armor is still pretty damn good for what you’d need it for: proof against most stabbing weapons and edged weapons, decent against many missile weapons, some protection against impact weapons.
As Kinthalis pointed out, you would be toast on a medieval battlefield. Even if they couldn’t get through your armor, and your joints were all well protected, no armor is proof against getting mobbed, knocked off your horse and run over, or just having your joints broken or dislocated by wrestling techniques. I’ve been doing martial arts, including weapons work, for half my life and I still would probably end up dead or incapacitated in just a few minutes.
You just can’t train as hard and don’t have access to the quality of opponents and instructors in modern life as those guys. They started training when they were kids. It would be like Joe Average going up against a professional gymnast in an acrobatics contest. With that kind of disparity in skill, gear doesn’t matter much.
On preview: ivn1188, the difference is that those guys would have been training in those tactics most of their lives. In a game of basketball, you wouldn’t need NBA players to beat the crap out of a group of medieval knights you threw in your time machine. Most pickup basketball players would do well against them. Your group of modern army guys would not have the same innate grasp of how to function in that kind of conflict. Even part-time levy troops would probably have accumulated years of formal practice and drill instruction that can’t be duplicated in just a few months. I would not expect a group of professional soldiers or knights to have much trouble with them, absent technological tricks that they wouldn’t have encountered by that time.
Give the modern mindset for logic and empirical problem solving a little time to work, a few years for training and drill, and yes, modern guys would have a much better chance, probably even a temporarily unbeatable one. But the question was whether modern equipment itself would give an insurmountable advantage. No, it wouldn’t. Skills matter more than gear. Modern materials wouldn’t be hugely superior in comparison to what they had anyway, so you’d be at rough parity with respect to equipment.