How do I make a custom discord bot @ someone that a person @ed in the command?

When I type !best it comes up with my username which also does it if I @ someone else with the same command like !best @example and comes up with @nottheexample

if message.content.startswith('!best'):
        await message.channel.send(message.author.mention)

Answer

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.