Half-Life - You take control of Gordon Freeman in the Black Mesa facility.
This is assuming you want to follow the Half Life story from start to finish.
Half-Life Alpha developer playthrough on YouTube - Recorded from the included demo files.As already pointed out, the most logical order is to play the games by their initial release dates.
Half-Life trailer from GameStar magazine on YouTube - Features footage at 01:05.
OGR - Monster of the Week preview from 1997 on YouTube.
Half-Life: Preliminary Findings - Ken Birdwell interview on YouTube - Features footage at 00:24.
Half-Life: Preliminary Findings - Introduction on YouTube - Features footage at 00:19, 00:23.
The flash grenade is not present in any build of Half-Life. In the map c1a3b, there is a desk drawer with a developer note only visible in a level editor that states flash grenade in here. The player can interact with many props such as crates and drawers in the levels. While the chapter's general premise eventually evolved into "We've Got Hostiles", none of the levels would later be reused in any Half-Life game. The chapter was cut following Half-Life 's significant reworking in late 1997. From here, the controls can be used to shut down the Automatic Turret and unlock the silo door at the beginning of the chapter, allowing the player to return to that point and continue into the silo. A large battle was to take place between human and alien forces on the visible landing field outside. The door to the silo is locked, so the player must navigate into the storage areas, avoiding an active Automatic Turret and solving a puzzle to safely get through the air ducts, to reach a control room on the surface. The level begins in a corridor built around a silo. This chapter is set in a large warehouse setting.