The first one isnt really that good by todays standards, but it was revolutionary when it came out. The three second parts: hl2, hl2:ep1 and hl2:ep2* are really good. A little bit of physics based puzzling and good shooting. Really good voiceacting, cool characters and very engaging story. Also, it is amongst the best storytelling out there.

Valve takes their time making games and so they are all very good. HL2:EP2 has developer commentary and you should play through that as well after you finished the game once. It really shows how much work and effort they put into every little detail in the game. They especially seem to do tons of playtesting. Because of that the level design is spot on.

The designchoices in all of the half life games really set them apart from every game at their time and even most games of today. One of the main designphilosophies of half life is immersion. They never let the player character speak, so you can imagine your own voice, there are no mirrors where you can see yourself, so you can imagine your own face, and they never take control away from the player. The last one is especially important. There are no prerendered cutscenes. You are always in first person. There is nothing that breaks the relation between the player and the character in the world. Everything you learn about the world and all of the storytelling is done directly via experiences that you have in the game. Getting thrown over a chasm in a car by a robot? Youre in the car. Suddendly awake in the future? You know exactly as much as Freeman does.

So yeah, play them. They are pretty cheap, too. You can get the complete Valve Pack, so Portal, Portal 2, L4D, L4D2, HL, HL:Blueshift, HL:Opposing Force, CS, CSS, HL2:EP1, HL2:EP2 and so on for 90 EUR.

*ironically, they did episodic content so people wouldnt have to wait forever for HL3. At least thats what they said.