There is no legal mixer because the mixing service is based on anonymity, which is something that many governments do not prefer and do not give anyone a license to make tracking money more difficult.
OP isn't looking for legal, he is looking for
legit mixer. Legal and legit are fully different. If there is a legal Bitcoin Mixer then it will more worst and I don't think any legal authority will give a certificate to run Bitcoin Mixer.
Anyway, the thread above the list of Bitcoin Mixer is enough to choose the best one. You may compare each other and choose what you think is right. Good thing you asked here before getting a scam, so you are in the right direction.