GW or FFG/Asmodee (aren't they the same thing now?). FFG because of the varitey... simple games, party games, Euro's, AT... the whole shebang. I like Stronghold too.
Since I only get one answer it'll be FFG. GW is close but there isn't quite enough variety. I have no clue what I'd play when my Wife and Dad want to join in.
GF9 would never even enter my mind because from what I can tell all they do is licensed properties and those do not interest me at all.
GMT, no contest. Between the wide variety of their catalog to the depth of some games ( Combat Commander has tons of scenarios as well as a quality Random Scenario generator ) its an easy choice. Just all their card driven games alone would be enough. As a wargamer, MMP would be tempting, but too much of their stuff is longer games, whereas GMT has a ton of single session stuff .
I would have said FFG at one point, since I still have a large selection of their games. But GMT has grown on me in the past few years and those are the ones I really feel compelled to play these days.
Looking in my closet I can tell 85% of my money is in FFG games so I'll have to go with them. They don't always get it right but at least they're always trying to make the kind of games that I enjoy. Which are mostly non-euro, sci-fi, fantasy, horror, and other war-like thingys.
It's a toss-up for me between FFG and GW. Runner-up goes to either Plaid Hat or Sierra Madre.
I can't actually make a decision and choose one, but if every game company on earth disappeared except one of those four I would still be a happy gamer.
GF9 doesn't make the cut only because they don't have the depth (yet). GMT doesn't make the cut because they really only have a couple titles, namely Leaping Lemmings and Conquest of Paradise, that I keep going back to.
FFG and GW are a toss up and I would be happy with either. If I had to choose it would be FFG as it has a few games I could use to replace similar stuff I own (eg GW has no deck builders as far as I am aware).
Rio Grande is one I haven't seen mentioned yet, but I own a surprising number of their games. Galeforce9 are good, but not enough variety.
It's really damned close between GF9 and GW for me. GW would provide a lifetime of dice chucking fun, and I really love the 40k universe. On the other hand, GF9 isn't afraid to shy away from dice chucking or more old school design, but their games are *smart*. I don't think I could do without Firefly, ST:A, or hell, WWE. GF9 might have the edge for me.
The thing about GW is you get more than just the game. You get the stories, basic rules that are usually getting refreshed, painting, building, army development, several types of play, board game options, money-spending...what you don't get is anything other than wargaming. So no economic games for example.
But I'd actually be kind of OK with that. Being focused on the "GW hobby" would probably be more satisfying to me in an environment where we are getting all of this Kickstarter junk, repetitive concepts, lots of games trying to simulate playing GW games...
10 years ago I would have said FFG. 20 years ago Rio Grande.