Also, from Feynman lectures, which are very easy to read through, you can see it´s in line with Microwaves101: see formula (24.19), you can find the lecture on this link http://www.feynmanlectures.caltech.edu/II_24.html

Hi Stich,
if you don´t have space or weight restrictions, then I would suggest a cavity / waveguide filter to achieve the sharpest response. Actually, as you go higher in frequency, the filter gets smaller and for high frequencies the size and weight may be comparable to other technologies. Every technology has its advantages and disadvantages, so usually you should look at your overall requirements and find the right balance (like Jonathan is saying, you must have other requirements in mind: IL, Power handling, size, weight, rejection, ...)
regards
AA