Half life
Half life
Half-Life -> Half-Life 2 -> H-L2: Ep1 -> H-L2: Ep2 There's also Black Mesa, which is a independent dev-team's retelling of the events of the original Half-Life. It's really good, though it has bad voice acting and the human character models seem to all have hunchbacks and too-long arms.
I believe back in 2004 they intended Half-Life 2 to be a standalone title with plans for a future entry, Half-Life 3. However, the strategy shifted over time, leading to delays and the release of two expansions for HL2—Episode 1 and Episode 2. Episode 2 concludes with a major cliffhanger, suggesting they still aimed for an eventual third installment or a full game. We don’t know the final outcome. If you wish to experience Half-Life 2, the correct sequence is: HL2 > HL2 Episode 1 > HL2 Episode 2. You can purchase the base game separately. Playing the episodes follows naturally from the previous one, so it’s best to follow that order.