Seconding Omega's recommendation but as you can see, we're both carrying Chipmixer services in our signature, so you might say (quite rightly) that we're biased but I think there's nothing wrong to recommend a service that we use, like, and are willing to stake our signature reputations on.

But to answer quickly why I think it's good:
- it was the first to introduce  "chip" denominations, where private keys to pre-loaded addresses are given to you (as private keys) to use at the moment you need them. This was an innovation in mixing because using these chips means you use coins older than the deposits you send for mixing, making it that much more difficult to link your deposit to the chips using linear time.
- you can further add randomness by choosing to either gamble (provably fair) or donate (yep, you choose the fee you want to pay) chips, resulting in a final balance that is different from your deposit.

No other mixer had that before, am doubtful there's one that's imitated it either.

Final note: you should never trust any mixing service. You should choose one with a good reputation and one that meets your mixing needs. Visit the threads on this forum in the Services section. You'll get an idea of how good/bad people find their experiences using different mixers.