I found the expansions of Half-Life very interesting because them shared the same story, and happens at the same time, but with different point of views, also in every expansion there are visual connections to the main game (or to the other expansions), so you can understand better what is happening in the main story while your actual character is doing alternative things.
Half-Life - is the POV of Gordon Freeman.
HL: Blue Shift - is the POV of Barney, the policeman that can be seen punching a door at the start of the main game.
HL: Opposing Force - is the POV of one of the military men sent to attack the laboratory.
There were also different great fan projects that continued the trend, I'll list only the best two that I remember, but there were more:
HL: Point Of View - is the POV of one of the aliens that appears at the laboratory, and it's DEEP, damn this game adds a lot to the lore, even more than the official expansions, it starts in the alien world and you can see how it was managed.
HL: Azure Sheep - is the POV of another Policeman trying to save a colleague. It has a connection to the previous "Point Of View MOD" as them share some same locations where, well, in both MODs you can see the other protagonist from the other point of view ...