The Legend of Zelda

Some series have a good list but with a couple of bad games (Tales of D: ) so it does bring down the overall series. Disregarding spinoffs, I've never played a Zelda game and thought, wow, this isn't fun. Sure some are more fun than others and others do have mind numbing sections or annoying features (motion controls) that makes it difficult to play. But overall, the games are still fun.
That said, I'm the type of person that has trouble going back to the older games after playing the new ones so I haven't gotten far in the original Zelda and haven't played Zelda II at all. Currently going through the Oracles and they are pretty good

Fire Emblem and Rune Factory are probably game series after Zelda.. might explain in more depth later

Hitman

Yes, back then I played anything Disgaea related.  Not as much now, but PS2 days I was bit obsessed with the series and style.

Dark Souls

Dragon Quest

Final Fantasy

Deus Ex

Zelda

Shin Megami Tensei

Fire Emblem

Uncharted

The Elder Scrolls and Fallout

Persona
Uncharted
Dark souls
Fire emblem
Romance of the three kingdom
Civilization
Europa universalis
Elder scrolls

So many anyway now,

1. The King Of Fighters
I love the concept of playing different characters in a single fight. I was a lot of fun in 1994 and it´s still fun today. I love the characters. The only thing that pisses me off in the series are the cheap bosses, but I kept loving that series like no other. I played most installments on arcade and some later installments on my nephew´s PS2.

2. Shin Megami Tensei.
Actually, I just recently discovered that series because some years ago, I had only Nintendo consoles and I wasn´t a big fan of portable gaming (gaming is very expensive in my country and at that time I couldn´t afford to buy and follow more than one console). I got a 3ds in 2011 and after playing Devil Survivor Overclocked, I just couldn´t get enough. I bought every Shin Megami Tensei game launched since then and now I´m looking for older games in emulators. I´bought a PS4 so I can play Persona 5 when it launches. I also bought Tokyo Mirage Sessions for WiiU

3. The Legend of Zelda.
Played it since teenager, since the first NES game, with my older brother. I´ve not played all main games in the franchise but I love it anyway. No fantasy game caught my imagination as strong as Zelda series did. And we still play Zelda together after so many years.

4. Professor Layton
Again, a series that I only discovered recently. Love the characters, the stories, the concept and those puzzles are a lot of fun to solve. And that Layton vs Ace Attorney crossover is amazing.

Other series that I love but not as strongly as the previous four: Animal Crossing, Street Fighter, Mario Kart, Virtua Tennis, Resident Evil, Just Dance (best party game ever!)

Battlefield, GTA and LoZ

Uncharted
Metal Gear Solid
Arkham
Mario Kart
Zelda

Thanks!

Did you like the more Metroidvania-esque Arkham Asylum or the open worlds in City and Knight better?

Metroid- without going into too much details because I could talk about it forever there's a lot of things that make metroid special to me:

It got me into gaming- even thought it wasn't the first game I ever played it was the first as a "gamer".

It's the most inmmersive franchise I've played- I can't quite lose myself in other games like I do with metroid, the music, the enviroments, the exploration of the uknown, the loneliness really make me get so invested in the game.

I never get tired of playing it- so yeah I already lost count of how many times I've played the prime games, I've also replayed super metroid and metroid fusion at least 3 times.

My favorite game of the franchise is metroid prime 2 and well I don't really want to talk about the current state of the franchise...

I'll add other series later.