To mention a user you have to define it beforehand. You do this as follows:
user = message.mentions[0]
To mention the user you can either use f-strings or format.
Based on the code above here is an example:
@client.event # Or whatever you use
async def on_message(message):
    user = message.mentions[0]
    if message.content.startswith('!best'):
        await message.channel.send("Hello, {}".format(user.mention))
Please note that the code only works if you then also specify a user. However, you can add more if or else statements if you want to handle it differently.
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solved How do I make a custom discord bot @ someone that a person @ed in the command?