It stands to reason that the best interfaces are the ones that you can use (or learn to use) with the least amount of thought. In other words, the ultimate interface is one which you can use without explicit thought at all, at which point the interface ceases to be an interface, it becomes invisible.

The way to achieve Man-Computer Symbiosis is by continued innovation in interfaces, bringing the abilities of humans and machines closer and closer until the interface ceases to exist. Until the human and the machine are one and the same.

Any current interface is grossly ill-equipped to handle this, the mouse and keyboard are both bandwidth-limited and unintuitive interfaces for communication. Touch interfaces are slightly better with their fluid interactivity. Voice interfaces are even better but inconvenient and unnormalized for use in public settings. Thought interfaces such as EEG systems are the holy grail here, but are far from reality.

It is worth noting that the mouse was invented in 1964, and half a century of digital innovation later, remains mostly the same.