HL greatest single player game ever? PLEASE...it took me 4 years to finally get enough desire to play through it all. And after getting half-way through, I just wanted to get it over with. There wasn't much of a story to brag about...I only finally understood what the last enemy had to do with the whole "experiment" after I read the FAQs out there. That's what I call a serious flaw.
MP is its strong point- although it's quite dated, at least 9 out of every 10 PCs in the world should be able to play it with a decent frame rate. It still holds its own in the MP world compared to old school Quake MP (which also had its merits).
As for single player experiences, Deus Ex, System Shock 2, and NOLF are my all-time favs (and although I'll probably get flamed for it, Sin was a guilty pleasure because of the locational damage)...even though they had cutscenes, they didn't detract from the experience at all. Nothing in HL came across as revolutionary. These games all added something to the genre.
To each his own...