A LOC diode (which stands for Long Open Can) is currently the strongest diode I know of that can be bought at what a hobbyist might consider a reasonable price. The new 24x thats coming could have a 5.6mm diode in it but nobody knows yet.
There are so many manufacturers and distributors that keeping track of the most powerful would be impossible. I would not be surprised to see half a watt coming from a 5.6 diode but you might be looking at several thousand dollars.