I'm going out on a limb here and going to say Valve is not so great; this half-life story has gone on for way too long (and trust me, will be a major disappointment when it finally hits, along witha few gigs of nearly identical textures and the same gameplay from four years ago), TF2 isn't a great as everyone hypes it to be, and CS is the single most overrated video game in history (bad hitboxes, non-existent hacker prevention, amateurish rendering, horrible bandwidth usage, camper friendly...why do people play this rubbish?)
That, and their decision that to play anything from them I have to inject my system with the bulky, slow, buggy Steam nonsense. This is the reason I keep Quicktime and RealPlayer off my HD, I don't want yet another memory hungry background app eating my clock cycles and bandwidth. Not to mention the havoc these things wreck on the already disastrously conceived Windows registry. Played HL2 (+episodes) and removed the entire install once I finished.
But, it's a solid company, and can manage a decent release every once in a while. But greatest? no.
Blizzard is ok, but got way to side-tracked with WoW, which after some initial success managed to run the whole thing into the ground...and keep running it into the ground...over and over. Hopefully SC2 will bring back the magic.
Maxis(back in the day), Stardock, Id (back in the day), Capcom, Nintendo, SquareSoft/Enix are/were all great companies. Better than EA,Valve anyway.