So I played HL1, OP4, BS, and Uplink last winter and just got around to playing the HL2 anthology which I beat in the last week and a half.
I have yet to play Decay but I don't think that that's that big of a deal.
And man I wish there was more out there cause the way the story ended just makes you want to know what comes next.
I might make a couple of posts on my general thoughts on each game but I do think that Half-Life is the best cause it doesn't make you wait for people to talk every chapter which can get very annoying on subsequent playthroughs like in HL2 and has better level design and length than OP4 or BS.
Don't get me wrong I love how the story is handled in the HL2 anthology it just gets annoying if its your 3rd time hearing the same conversation while you are stuck in a room.
Anyways I'll leave you with this: