Okay I just finished Black mesa and to be honest it's pretty challenging , even on "normal" setting. But everything is so detailed - the models , the voice acting , the missions , the scenery hell they even included the healing soda cans of the vending machines.
They even referred to an older concept of Gordon Freeman - after leaving the tram after the intro and getting your door opened , the security guard in the checkpoint says something like "Aw man you cut of your ponytail".
