You do know Valve exists outside the Half-Life franchise...
So, let me tell you my thoughts on the games:
Half-Life: You're right, shit by today's standards
Half-Life 2: A bit repetative, the story isn't told well, but I still think the hours leading to Ravenholm and after Follow Freeman were extremely fun. Innovative engine, nice puzzles. Also the extremely large modding community.
Episodes: Straight into the action, but not many new features
Counter Strike: Replayable, fun, worth the money. However, hard, frustrating, full of cheaters/exploiters, and the sniper rifle.
Day of Defeat: Never played it
Left 4 Dead: Sure the zombie franchise is overused, but there are many things taht made it unique: it's multiplayer, teamwork needs, franticness, and able to play as the infected. Gets repeitive after a playthrough or two.
Team Fortress: Haven't played Classic, but number 2 is in my opinion the best online First Person Shooter. Balanced, tons of free maps and updates, hilarious, and replayable. It's a bit complicated to understand and is hard.
Then again I'm a Valve fanboy...
But the main reason I love their games is that they're innovative, imaginative, atmospheric, immersive, etc.