Sasuke is pretty mentally stable, at least judging by that, sure. However, Kakashi, Gaara, Itachi and Naruto put up a great challenge.
Naruto had to grow up with everybody hating him and he did not turn into a blood-thirsty killer. Props to him. Itachi murdered his own family, which is much worse than just watching it die, even if it's for three days in a row. Oh, and he had to live a lie his entire life and never looked unstable, not even for a second, unlike Sasuke. Gaara, meanwhile, was mentally unstable (very) for some time, but he ended up overcoming what was probably the harshest (maybe on par with Itachi's) of background stories in the Narutoverse. He ended up becoming Kazekage.
However, I think Kakashi takes the cake. He killed Rin, yeah, but Itachi did something far worse and on purpose. However, what I think demonstrates he's mentally stable was that he survived Itachi's Tsukuyomi. Imagine excruciating pain three days (or one hundred and eight depending on your view of it - if you're going by that, there's nobody close to him in terms of mental stability). That's worse than watching your whole clan die, and probably worse than even Itachi's tragedies. Kisame was surprised Kakashi was still alive.