Half-Life: Opposing Force, of course.
To be entirely honest, I actually like Opposing Force slightly more than Half-Life 1. It's like a shorter but sweeter version of Half-Life, that adds all sorts of cool extra features to that original formula while also stripping out some of the more obnoxious stuff (there's significantly less platforming than in Half-Life 1, for instance). Whenever I find myself in the mood for a nice old school shooter, Opposing Force is usually the one I'll turn to, especially now that Black Mesa pretty much makes Half-Life 1 obsolete.
And speaking of Black Mesa, here's hoping that Operation Black Mesa [http://www.moddb.com/mods/operation-black-mesa] can be nearly as good.
And speaking of Black Mesa, here's hoping that Operation Black Mesa [http://www.moddb.com/mods/operation-black-mesa] can be nearly as good.