Q.U.B.E.
It's clearly derived from Portal, it has no plot or storytelling whatsoever, it has one of the least sophisticated menus of any game this century, and it seems to be pretty dramatically short. But to wholly dismiss QUBE on these demerits would be slightly unfair, because its puzzles are (after the first few) genuinely clever and mentally challenging. QUBE's pacing is also legitimately good, as new mechanics and twists are introduced quickly enough to keep puzzles interesting, but not so quickly that they ever seem insurmountable.
The soundtrack is pretty neat, too.
Progress: Sector 5