Broken Sword
Dragon Age
Elder Scrolls
Gabriel Knight
Gothic
The Witcher

5 is just not enough:)

1. Ultima (5-7,Underworld 1+2)

2. Monkey Island (1-3)
3. Fallout (1,2,NV)
4. System Shock (1,2)
5. The Witcher (all, but continuously improving from 1 to 3)

But really, ranking my favorite series is difficult. There are so many awesome series! Max Payne, Portal, Baldur's Gate, Gothic, Elder Scrolls, Descent, Wing Commander, X-Wing, Dark Forces, KOTOR, XCOM, Prince of Persia, ...

But not Deus Ex or Mafia though. Both were just one awesome game with mediocre successors. That doesn't make it a great series.

-Heroes of Might & Magic (Come on, guys, how am I the first to bring it up?)
-Monkey Island
-Deponia
-Max Payne (though I have not played the 3rd one)
-Dark Forces/Jedi Knight

Not on the list
-KotOR (because as much as I still love it II was basically unfinished, and the MMO Old Republic left me with a bad last impression of the whole franchise)
-Warcraft (because fuck WoW and Hearthstone even more so)
-Disciples (the first two are easily a match for HoMM, but the third one ruined it all)
-Divinity (it's a rollecoaster of great, fun but flawed and borderline unplayable games)
-Fallout (just my personal bias, the games fro mthe series I played were good or even great, and yet I can't quite fall in love with it)
-Batman Arkham (becaue... actually, maybe I should put it there?)
-Witcher (have not played the third one yet, and the second was a huge disappointment)

I have played all Fallout games but haven't finished any one of them lol. It's just isn't my taste. Other people really love this series though.

Civilization
Borderlands
Railroad Tycoon
The Witcher
Bards Tale

I'm surprised you haven't played the third part yet. Give the game a try and I'm sure you will love it. It's much better than The Witcher 1 and 2. It also has an open world filled with so many side quests (not repeated quests like The Elder Scrolls series. Every side quests in The Witcher 3 is unique) unlike The Witcher 2 which was a linear game.

1. Mass Effect
2. Final Fantasy
3. Deus Ex
4. Metal Gear Solid
5. Resident Evil

Top list
Monkey Island
Mechwarrior
Myst
Commandos
Commander Keen

And if I could add more
Descent
Command & Conquer
Age of Empires
Need for Speed (up to the first Most Wanted)
Dark Forces

I'm sure I will, but first I have to get a new PC for it :D But seeing how the vast majority of games on my wishlist will still run on my trusty old rustbox, I'll be sticking with it for the foreseeable future.

I am going to pick five great game series, which have at least three games, in no particular order and maybe not really my favourites but I find them very good:

Samorost
X (Egosoft's)
Unreal
S.T.A.L.K.E.R.
ARMA

That's a really difficult question. In part because it's difficult to find really good series, and in part because I haven't played most of them.

Instinct says Deus Ex, but in fact I've only played the first two.

Maybe System Shock, but in fact I've only played the second game.

Doom would probably be one of the strong contenders, but truth is I don't really love the stock Doom 2 maps, TNT kinda sucks.. Doom 3 is great yet not a masterpiece. I haven't played Doom 2016. I love classic Doom but mostly for the community works it spawned. Still, a damn strong contender?

Need for Speed? I've only played Underground + Underground 2. Both really really nice games. However there are many titles in the series I feel like I wouldn't like that much. Still, really solid.

Hitman is probably the one I'd consider the best in that the first two games are almost 100% solid and the little I've played of Contracts (the demo only) is also 100% solid. I'm a little hesitant though because I haven't played the newer titles.. except a bit of one, whose name I do not remember.

Then again, the first few CoD titles are pretty solid too.

The first two Max Paynes are really solid. Can't remember if I did the third.

I don't know man, I really don't. Games suck donkey.

Pulled out of some part of my body quickly:

Mario. (Seriously. Referring to the platformer core.)
Civilization.
Castlevania. (RIP.)
Mechwarrior. (RIP.)
"Racing games by Eden Games" (various actual franchises)

Dishonorable mention: Myst sucks. Always has, even when it was new.

Deleted

I feel there can be a bad apple in a series for it to be mentioned here.