I have been dealing with bitcoin for more than 3 years 

Hello. Personally, I chose a btc mixer for myself according to the following criteria:

Level of anonymity: How anonymous and privacy-respecting the platform is.

Reputation: A Company’s already established reputation and trust.

Fee: The amount of fee required (if any) for mixes and the users' control, or the lack of it on the fee.

of Additional Addresses: These are the number of addresses you can set for your Bitcoins to be received at, your coins are not sent to one address but are broken down and sent separately to each specified address further fortifying your anonymity.

Advanced features: Support for advanced features such as Delayed output, mixing code etc. Delayed output is the feature which lets you receive your Bitcoins not instantly, but after a time gap which you can set, this again is a feature which de-links your input of the coins, and their output.

Minimum / Maximum amounts: Any mixer which requires a very high minimum deposit in order to mix your coins won’t probably be a good option, which we do consider while selecting these.

Confirmation Required: The number of confirmations required by the platform before they mix your coins, the lower this is the faster the overall process will be.

No Logs Policy: If the Bitcoin mixer keeps logs, and if it does then for how long.

I believe that covers nearly every feature a Bitcoin mixer may comprise of, or we need concern ourselves with isn’t it?

Therefore, I advise you to pay attention to those mixers that fit these criteria.