Tumblers have come a long way and the best are those that completely mask the identity of the user by dissociating the input wallet from the receiving wallet. This is accomplished by creating a time lag from the time the coins were sent and the time fresh coins are received by the bitcoin users.
Bitcoin blenders use a unique technology to break the connections on the blockchain. This means that blockchain analytics can easily monitor the transactions and possibly know the identity of some users.
The best onion bitcoin mixer has enough coins in its reserve. These coins that have no previous link to their wallet would ensure that their anonymity is guaranteed.