It's a hard decision between Half-Life 1 and Half-Life 2 for me, Half-Life 1 introduced a lot of innovations and immersion methods never seen before in video games including scripted events and a protaganist that is directly influenced by the player. Not only that but on how cutscenes are directly seen in gameplay by the player, and the shooting mechanics for its time was amazing as well, and has aged well thus is still enjoyable by today's standard.
Half-Life 2 is definitely a ground breaking game during its time with a lot of advancedment in graphics and physics. The introduction of the Source engine's capabilities proved to be not only as eye candy and physics machine, but as a open very easy to mod engine making it possible for fan-made mods and maps. Along with the gravity gun, being one hell of an awesome weapon both fun and effective.
Overall I think Half-Life 2 amazed me the most when it first came out and exceeded my experience of Half-Life the first time by a thin margin.