Half Life 1 was a very popular video game, and was used many times to make machinima. But it is considered by everyone that Half Life 2 has doubled, maybe even tripled it's efficiency for machinima.
Because Valve have given access to mod the game's engine, many "modders" and programmers have taken advantage of this feature to access the game's engine, known as SOURCE.
With a quick click on the steam website you are able to download a program called Source SDK, which I believe stands for Source Developement Kit. From the menu you can access the "Hammer Editor" application, which is the name given to a level editor app. You can also use "Model Viewer" which speaks for itself. And lastly you can use perhaps the most useful application in the machinima community which is of course is "Face Poser", which you will need to use IF you want to make good quality animations for your films.
You can also create mods which are without a doubt useful if you want to improve some features of your movie etc.
Many good quality films have been made using Half Life 2.If you go to machinima.com and into the Half Life 2 channel, some great movies have been made. My personal favourite is "A few good g-men", which was done using Half Life 2. But if you don't have the time to learn Source SDK and all that techno wizardry, then you can purchase Garrys Mod, which is a very well made mod for Half Life which is considered to be a "ture sandbox" game, and perhaps in my opinionn the best sandbox game out there. You can spawn objects, weld them together, freeze them in mid air, position ragdolls in certain poses, nearly anything, as long as your imagination is of good working order. Garrys mod is another frequently used mod to access levels and re-position everything with ease, and also spawn more things in to make their movie truely epic. The developers console in Half Life 2 is also essential for movie making, if you cannot afford some type of recording application to run in your game (fraps etc) or simply don't want to use a free application that posts ads in the corners etc, then using the in built demo recording system of Half Life 2 is very useful too. You simply go into the developers console (activated by pressing ¬) then you are able to input the console command to start recording.Whilst this sounds very easy and usefull, most people do not use this method and go for fraps, which then if downloaded for free will place fraps.com at the top of you movie clips. Most people will crop that out though.But it is highly stressed by many that you use the demo recording system built into Half Life 2, because it's already there, so there is no excuse. So there you have it, Half Life 2 is the best one to use, Half Life 1 can also be used, but might prove more troublesome or awkward since Half Life 2 has been simplified and made more user friendly for things such as machinima.
The graphics are beautiful.At the time of release these graphics were truely amazing, and a few years later, they still are to most people. The physics system is one of the best out there, if not THE best, and with more episodic content such as Half Life 2 : Episode 1 etc coming out, the modding community will only release spawn menus for garrys mod, which will allow you to use more models and newer levels. The possibilities with this game are near endless...