Half-Life 2 is a monumental and entirely memorable game. It is one of the few, in fact, that I can recall every single situation from, even though I haven't played it nearly as much as some others. I have to give it the nod for some of its more experimental features, as well, like the Highway 17/Sandtraps chapter, where Valve tried their hand at open-ended exploration and did an excellent job of it, in my opinion, while still keeping the player on track towards the goal. The expansions lack some of that freedom and emergent storytelling.
Half-Life, though excellent, has some design flaws and dated gameplay that have become quite apparent over the years - still a lot of fun and holds up excellently, but it does not feel particularly modern and there's some frustrating parts here and there. Its influence, of course, cannot be understated, and was probably the first game I ever played that truly felt "real" to me. The visuals at the time were amazing, and the world itself beautifully realised. It seems obvious today, but navigating a science lab and military camp was a huge step above the drab, repetitive Gothic castles of Quake.
From a design perspective, I'd say Episode Two is the best, simply because it's the culmination of Valve's single-player design work up to this point. Excellent character acting, moody environments, enjoyable puzzles, great action, and a fast-moving plot with lots of variety in settings and gameplay. It never drags or gets boring - I can play it start to finish in an afternoon no problem, and enjoy it every time.