A fitting OST is what makes a gaming soundtrack special. I actually have a lot of songs unrelated to gaming whose brings back to me special feelings because many many years ago I played them into background while playing F.E.A.R., because I was too scared to play it otherwise. That was when I had a pretty shaky mentality and was easily influenced, so I used it as fix. Also there are a lot songs whose for me personally are special, because I remember from hearing them originally from fragmovies, so hearing them makes me remember feeling of excitement I once felt while watching them.
Therefore I don't think that justice can be done just by posting high quality songs from good games, you have to experience those games yourself to enjoy them to their full potential. Awesome songs like Half Life OST is something everyone could relate to, since everyone have played HL or at least most of us.
I still often play a random playlist while playing casual games like Borderlands and sometimes I end up getting hyped up not for game itself, but because a song starts playing which brings back memories.
But as for original gaming OST, my favourite is OST of s.t.a.l.k.e.r. series, because it's for me one of most atmospheric games I've played and it's soundtrack is a great reference of that experience I once felt. Not anymore however, I no longer feel as excited when playing scary games, so I guess this is why I've choosen as my favourite OST, since it reminds me of something I've lost.