Fire Emblem
Paper Mario/Mario & Luigi

The only 2 RPG series that I have played more than one game of(that I can remember).

Souls
Elder Scrolls
Pokemon

1. Mass Effect.

That is all.

FF, but FF of old.
Fallout
and most importantly, Dragon Age, I love the universe they have created.

Fuck this is really fucking hard, as an RPG fan I can't really do this

I guess Xenoblade doesn't count so I won't do that even though the next one looks amazing and I'm sure it'll be one of my faves...

hm...

Ys
Fire Emblem
Tales of

Honorable mention to The Legend of Heroes: Trails series (a sub-series of The Legend of Heroes). TWEWY would be in there if it were an actual series or franchise or whatever, lol, can't really consider it one though. And there are a ton of other RPGs I liked, but as far as big series goes, I guess I can bite my tongue and accept those 3...

Pokemon, Tales of, Fire Emblem

Hmm... I don't think I have a favorite RPG series. I play a ton of RPGs, but I don't think there is a series that I prefer. Many RPGs are stand-alone games. Some of my recently played games that come to mind are: Dragon's Dogma, Radiant Historia, Kingdoms of Amalur, Ni No Kuni (I sooooo loved this game).

But I also play many RPGs that are in series. The thing is, often for these RPG series, I've only played one game in the series. I love Skyrim, but I wasn't the biggest fan of Oblivion. I'm also playing Dragon Age Origins, and it's good but I dislike the combat system. I haven't finished Fallout 3, but so far I think it's a great game. I've played FFXIII, but that's the only FF game I've played. Oh also Dragon Quest IX on DS was really good, I played that a ton. Then there are some other RPGs I have played but not completed: Torchlight, Dead Island, plus all the other ones that I have yet to start (thanks, Steam sales)

The Demon/Dark Souls series is one that I really like. Again, I've completed Demon Souls, but not Dark Souls (which is near completion... I think). This series has really superb gameplay and that is one thing I absolutely love about it. Borderlands is a pretty good series, if you would count that as an RPG. Of course pokemon is a pretty good series, but it's really more of a casual kind of RPG. 

Western RPGs are probably my preffered style, but I like a good JRPG every now and then. I'm not too picky when it comes to RPGs. I'll try just about whatever. Except I refuse to play strategy RPGs. I don't like strategy games.

Mass Effect
Paper Mario

dunno what else

Final Fantasy
Dragon Quest
Valkyria Chronicles

Persona
Pokemon
Souls