They are both great but if you like mapping and modding I think Half Life 2 is better, mostly because of its better scripting and its better capabilities. But Half Life 1 is sometimes really goofy in a good way which can change the way something is.
I'm personally not one for playing sotry games over and over no matter how good, so to me the original will always have the edge of the extremely fun multiplayer it's just a shame that by far the most popular map on that is crossfire, for which I have a strong disliking. It would've been so much better if it was a 4 team TDM or something like that.