I guess I would have to go with Fallout. It is a series of detailed and deep RPGs (though shallower after #2; at least #3 still is an actual RPG, unlike Tactics).
Overall though, I'm not a big fan of long-running series, in part because I like some closure and transitions in story-heavy games (For example, KOTOR: it could have easily ended conclusively after the first game. However, the second game still flowed from the first, and came to another conclusion, and doesn't need another game to finish the story. And, honestly, I don't know where they'd go from there if they tried). It's nice to see original games since they have to innovate, and can experiment with new mechanics or just focus on the story element. There's no need to develop everything from scratch, but in theory the writing is the easiest part so it should be as hard to make original story and characters. I know game developers aren't writers, but there are a crapload of writers out there, and most of 'em are decent, so I'd think it would be worth it to hire a couple.
Too many games in the same franchise necessitates that they become disjointed, or connected only superficially, or in small parts (see the Zelda and Mario series). While that formula has proven it can work, it often comes across like you're playing an upgrade of your old game rather than an actual sequel. That's ok if you're not really here for the story, but if you are you'll have to do shrooms for anything to make any sense (and it still won't, but you'll THINK it does), or take it one game at a time and pretend all the others don't exist while playing the current one.