Without other details, it is impossible to answer correctly. In general, EN353 should provide higher hardenability (J7 = 34 HRC to 46 HRC) compared with 8620 (J7 = 25 HRC to 40 HRC).
You will need to consider section size, quench media, quench agitation, core hardness, case depth to name a few aspects. If your system will not meet engineering requirements you will then have to consider optimizing load size and even changing to a higher hardenability material.
What do you mean by "best material"? Can we assume these gears will be carburized? Depending on the specific requirements of your gear application, there might be better materials than 8620 or EN355 steel.
i have designed a gear box (tripple train) of 15 KW. its input is 900 RPM and output is 75 RPM. i just wanted to know the better material which can be hardened easily.
