At least with FF you get the online MMO games that will keep you busy in between 

Replay the old ones, watch porn, argue over who gets to be Purple Link in FSA. 

Metroid, everything else can go

Fallout. I can put hundreds of hours into it vanilla but on top of that is mod support, which can extend the game greatly.

Either Zelda or Mario, but I'd probably choose Zelda anyways =P

Pokemon would be the easy choice for me. The time I have spent playing Pokemon is easily more than every other series combined.

This.  Even if you only like a few of them, they have so much more variety in a lot of game design aspects than most series.  The good ones including some of the spinoffs will satiate your apetite for a focused narrative driven story, open world/semi-open world traversing, micro-managing, simplicity and complexity, depth of game design,  and for those of us that want high quality (the highest)  musical composition.  You can even grind to your hearts content or get lost in the minigames in quite a few of them.

Dissidia has your fighting genre covered (quite a well designed fighting game if I do say so, both for casual and hardcore gamers) with customizable characters in moveset and equipment.  FFXV will have your Action RPG/Western archetype tastes covered.

The FF series covers more bases than other series and at a higher level of quality in many of those aspects.

Metal Gear Solid.

Pikmin. Everybody I've showed that game to loved it. And I love it.

If I say Mario, does it count every Spin off as well?