What order should I play the Half-life series in?
The must-play Half-Life games are Half-Life 1, then Half-Life 2. The expansions and spin-offs for Half-Life 1 which were not developed by Valve are quite skippable; Opposing Force, Blue Shift and Decay return to the setting and events of Half-Life 1, but portray the story through the eyes of a U.S. Marine, a security guard and two scientists (since Decay is a cooperative multiplayer game), respectively.
The Source version of Half-Life 1 is, and I quote:
a digitally remastered version of the critically acclaimed and best selling PC game
Needless to say it's smoke and mirrors, there's absolutely no significant difference between the two.
My advice is to play the original Half-Life 1, then Half-Life 2, and if you'd like to extend the pleasure, sequels Episode 1 and Episode 2 for Half-Life 2 are quite good, but not as good as the two main games.
The HL2 episodes are not an accessory part of the story, but actual sequels. They do contain plot changing events. The same is not true for the Half Life "One" series, as far as I know.
Thus:
- Half Life
- Half Life 2
- Half Life 2 Episode 1
- Half Life 2 Episode 2
As for Portal -- it does contain some side references to corporate entities in the Half Life universe, but as it is that's more flavor than actual plot.
The Source versions are a remake of the original games on the engine used by HL2. Half Life: Source is pretty much identical to Half Life.
The recommended playing order is pretty much the release order:
- Half Life (Or Half Life: Source)
- Half Life: Opposing Force
- Half Life: Blue Shift
- Half Life: Decay (skippable - PS2 only)
- Half Life 2
- Half Life 2: Episode One
- Half Life 2: Episode Two
Half Life 2: Lost Coast is just a tech demo for graphics options that went into later Source games. If you want to be quick, you can skip everything but Half Life and Half Life 2.
Portal is also tangentially related to the Half Life plot, but isn't too important as it doesn't have much story of its own. Portal 2 may change this however.
As already pointed out, the most logical order is to play the games by their initial release dates. This is assuming you want to follow the Half Life story from start to finish.
- Half-Life - You take control of Gordon Freeman in the Black Mesa facility
- Half-Life: Opposing Forces - You play as one of the marines during the events of the original game
- Half-Life: Blue-Shift - You play as a security guard during the events of the original game
- Half-Life 2 - Years later you take control of Gordon Freeman in a new story
- Half-Life 2 Episode 1 - Continue as Gordon Freeman after the events of Half-Life 2
- Half-Life 2 Episode 2 - Continue as Gordon Freeman after the events of Half-Life 2 Episode 1
Someone has already mentioned "Half-Life Decay". This is a coop game mode available on the Half-Life PS2 release. It takes place during the events of the first game. It is possible to play this on the PC (as someone has managed to port it onto the PC).
Lost Coast is a very short demo. It takes part in the realms of Half Life 2, but from memory the location doesn't appear in the full game.
Half-Life also has a rich mod community. One of the stand out mods would be the Black Mesa project - whereby Half Life 1 is being remade into the Source engine, used to make Half Life 2. The difference between this mod and the Half-Life: Source release is that the graphics are being completely polished.
Depending on your tolerance, or appreciation, for older games, it is feasible to completely skip Half Life 1, or at least the expansions. They were great games but are still old games. Not to say Half Life 2 isn't an old game.
Will you miss anything not playing Half-Life 1? Not really. The story in Half-Life 2 is so different. A different time, a different location, different enemies... If Half-Life 1 never existed, Half-Life 2 would still be a great game and story.
If you play Half-Life 2 then the episodes are worth going through as they immediately follow events from the main game. There is also a cliff-hanger at the end of Episode 2. Episode 3 never appeared and people have been waiting a generation for the next installment.
Portal has been mentioned but the game is not like Half-Life. Don't assume that you MUST play Portal 1 and/or 2 because you played Half-Life.