My favorite series is Assassin's Creed. I've played every single game aside from Assassin's Creed III: Liberation. Only ones I haven't finished are Altair's Chronicles - which is borderline unplayable, mind you - and Assassin's Creed II: Discovery. You don't even have to doubt I'm going to delve into this fantastic topic.
First of all, I'm a huge history buff. Assassin's Creed features important periods in history, even shining light on topics that your textbooks don't mention. I actually played Assassin's Creed II when I was studying the Renaissance in World History. Needless to say, I aced it. It also introduced me to my favorite time period; the height of the Ottoman Empire.
I also love the story. Learning the story is arguably as in-depth as learning about the world of Tolkien's The Lord of the Rings. There are currently five novels, nine games, four short movies, one film in production, about nine comics, an online game, and two editions of an encyclopedia. Needless to say, there are plenty of topics to learn about.
The story itself is an adventure into the lives of Assassins whose lives have been touched by the evil tendrils of the Templars. Each Assassin has given his life for the final goal; to assist Desmond Miles in stopping the destruction of the world. Ezio sums it up perfectly when he speaks to Desmond - a mind-blowing moment for me, by the way.
Ezio Auditore da Firenze said:
Maybe you will be the one to make all of this suffering worth something in the end.
The characters themselves are also masterpieces. Altaïr Ibn-La'Ahad is easily my favorite character in all of gaming. Even some of the more minor characters - La Volpe, for example - are amazingly developed. I've grown an attachment with nearly every character that has been branded a name in the Assassin's Creed universe.
To put it simply, Assassin's Creed is my favorite game series.
I would say LoZ and Metroid tie for first, with Halo in a very close second. They all have great storytelling in the games and have great mechanics. Though it would be nice to have a new Metroid game soon.
I would diffidently choose Mega Man above all series it's so fun all the games have good stories it's fast past and full of action and it's one of the only gaming series I actually want to beat every game in.(I would want to beat every Pokemon game but I can't do that without beating Pokemon XD Gale of Darkness...witch I hate by the way.)
Legend of Zelda. (of course. Why would we be here if we didn't think highly of Zelda?)
Surprisingly not Sonic. I like Sonic Colors and Generations, but I've played a few more games since, and didn't like them as much.
Paper Mario. TTYD is my favorite non-Zelda game of all time.
Scribblenauts. I really liked the first game, and my newly-acquired Scribblenauts Unlimited is amazing. I love it, too. Haven't played the one in the middle, tho. I think it's Scribblenauts Unleashed, but that could be my mind messing with me.
...Does Solitaire count? I play way too much of that for my own good.
Hmmm, normally I would say Zelda(and that's true; it is), but that answer has mostly likely been used(it has been). So, I would have to say the Mass Effect series. I have never played a game where I actually felt the grandness of the threat on Commander Sheperd, and the universe itself. The time when you actually realize what's coming and that the end is inevitable is at the very ending of the series when you return to Earth. I won't spoil it, but believe me; it gives a feeling that will never be able to be recaptured in gaming.
Hmmm, this is tough. I really want to say "Legend of Zelda," being that I own four of the games and love them all, even if I love some more than others, but I have to say, Fire Emblem. I've beaten 2 of them (PoR/RD) and got to play 3 others, and I just really love them a lot. I like the flexibility you have for choosing which characters you want to invest your time into strengthening, I love their stories, and no character has ever tugged at my heartstrings more than Zelgius, which probably sounds weird, but it's true.
But Twilight Princess did introduce me to videogames, and I've played that one as many times as I've played Radiant Dawn. And then there's OoT, MM, TWW, SS... Yeah, LoZ's my second favorite.
My favorite game series is the Mother series, hands down. Fantastic musical score, fantastic characters, all around amazing story... I would even go as far as to say that Earthbound and Mother 3 are my two favorite video games of all time. I would say Mother, but that one was really hard for some reason.