The flaw with a mixer is that you are not anonymous to the operator of the mixer, so I would say that unless a flaw is discovered in Monero, it would be the most anonymous.
Well you are not anonymous to anyone after you have sent coins from your Bitcoin wallet. If people want to be truly anonymous they should not part with their Bitcoins and keep them locked in their wallets. And to be truly anonymous when trying to exchange Bitcoins people need to only opt for p2p real world transactions
wearing masks so their faces won't be seen. There you have the ultimate Bitcoin anonymity
And most certainly you will pretty fast end up in a police station handcuffed as a potential terrorist (along with your counterparty as an accomplice, lol). When they learn that you were going to exchange some bitcoins, your chances to break free will plummet like a sledgehammer on an egg. So don't expect anything good of it and get ready for unintended consequences...
Needless to say that being anonymous will be the last thing you'd be looking for then
