Halo of course

Warcraft. Ininite fun because of the level editor. If wow is counted too it'll make the case even stronger.

Pokémon. It beeing one of the biggest franchises in gaming means that this choice would be beneneficial for a big ammount of people interested in gaming.
Plus personal favourite franchise so...

"Final Fantasy" or "SMT + Persona" games.

halo

Gotta go with Zelda. If only that series existed Nintendo would have to make that glorious home console remake of Majora's Mask at some point

#Zelda4ever.

Hard one... Can't decide between Souls and GTA series.
But I think I will go with GTA series because you can enjoy it longer.

I would say xenosaga, still hoped for a sequel...

Soulsborne

I think people aren't thinking this thoroughly or rather go for a selfish choice that would seriously put a huge dampen on the gaming community. We would be talking about not being able to play anything differently except for what the franchise has to offer, and that is something that you have to really think through when trying to reply to this, not just your favourite franchise.

I'll talk about my case. Final Fantasy. Do I love Final Fantasy? Yes. Do I think it's the greatest franchise ever? No. Why Final Fantasy, then?

· Because it has over thirty games (quantity, so that people, at least, get a lot of games to choose from, if not play them all since there's nothing else).

· Because the formula is changed in every entry (meaning gameplay will be, in its majority, fresh between each entry, and there's something different in each case).

· Because there's a lot of spin-offs games that cover a multitude of genres (Shooters -Dirge of Cerberus-, Strategy -Tactics, Revenant Wings-, Racing -Bikes-, Rythm -Theathrythm-, the whole Chocobo games and the Tower Defense games -Crystal Defenders-, loot-based adventure games -Ring of Fates- and social multiplayer ones -FFXIV, Crystal Chronicles).

· Because there's something in every platform conceivable, meaning people would had multiple ways to get into this - only - gaming franchise.

· Because the merchandise from the franchise allows people of experiencing it in multiple ways than one: books, comics, anime, movies, figurines and all other kind of physical and digital items.

Now let's examine some people's answers:

- OP said Battlefield. Cannot stress just how bad this would be. Sure, FPS enthusiasts and FPS multiplayer gamers would have their time of the day, but what about singleplayer people? (Their only relieve would be Bad Company 1 and 2, which feature a solid and enjoyable campaign). What about people who don't enjoy FPS games? At least there's diversity having games set in almost every possible scenario, but this would alienate so many players it's not even funny.

· The Legend of Zelda. Now this franchise has had a long tradition and features many games. Quantity is assured, but on diversity lies the problem. We have two different versions: the isometrical, generally 2D games, and the 3D ones; both follow the same structure in most cases (There goes Wind Waker and Spirit Tracks to Combo Break player's traditional progression xD), and the gameplay is generally the same as well. There's some small side adventures like Hyrule Warriors for the Dynasty fan, and...eh...Link's Crossbow Training for those who want to go back to the on-rails day. I can think of one social adventure game (Four Swords, there's that Tri-Link game on 3DS as well), but the rest would be purely exclusive to a singleplayer affair that is, at its core, unchanged in most entries. While certainly a better choice than Battlefield for singleplayer gamers, this franchise would really alienate a lot of players as well, so it's not that good of a choice.

· Fallout. It sure gives hundreds and hundreds of hours, and while 1 and 2 are radically different to 3 and 4 (meaning there's that fresh sensation to experience), quantity isn't assured in this case. There are some side projects like that strategy game where you're a Brotherhood member (can't remember the name, Tactics it could be?), but you'd be mostly left to those four mentioned games. Mod support can take 3 and 4 very far, but to be fair here, unless you truly enjoy the core mechanics offered there isn't much here for most players, and no way for social gaming unless some miraculously multiplayer mod like the one in Just Cause 2 is added, which isn't happening anytime soon.

· Metal Gear Solid. This franchise would simply left aside a huge percentage of the gaming population. While a gaming favourite franchise, with a moderate/huge success in most cases, it would be hard for most people to get into some espionage games on which you tackle down huge mechanical monsters after the President of the United States touches your crotch to confirm you're a man. Just saying. Not to mention replayability isn't all that great in most cases, although there's a fair percent of social gaming here counting the spin-offs.

· Souls/Bloodborne. Talk about the epitome of alienation.

I think Mario would be another great franchise that cover most bases, if lacking any kind of massive multiplayer online, but still a much better choice than many others.