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.