The Orange Box for me. I still don't really care about TF2 (just because I don't like online shooters) and the only Valve products I knew so far were Half-Life 1 (which I found just awful when I played it) and Counter-Strike, combined with Steam, I just wanted to avoid it completely.
Also, Portal seemed just like a ripoff of Narbacular Drop.
The I bought it...
And it was AWESOME! I loved HL2, I loved Portal, and I liked the TF2 design and "Meet The..." Videos (still missing the Medic though...), but still never actually played TF2. It might also be awesome, but as I said, I don't like online shooters.