The 9-level SP episode for Quake that got me my first job in the gaming industry. Back in its days this got over 10000 downloads in the first two days. There’s also various speedruns available. Start map, seven regular levels and a secret map. More information and links to level reviews can be found on Quaddicted.

Download Beyond Belief.
Download the Beyond Belief Prog Files for Tenebrae.
Download the Beyond Belief .map files (can be loaded in any Quake editor).
Download the Speed Beyond Belief demos (BBelief completed in 4:25 minutes!).