The best one is the one you prefer the most.
You could use the Facebook commenting plugin — then people won’t have to log in and can use their Facebook ID, and it could show up on their walls.
You could use Disqus, which has many features and is quite robust.
Or you could use the one that comes with WordPress, that way you’re not depending on 3rd party services and you’ll have full control over the look and feel.
And so on…. So yea, it really depend what you want in a commenting service in the first place, the best thing is to just try and see.