Command and Conquer & Red Alert - until most of the people from Westwood were driven away by EA.
Heroes of Might and Magic - until Ubisoft took over
Kohan - PLEASE GOG!!! Get it here, I'll buy it day one!
Caesar/Zeus/Pharaoh - until most of the people from Impressions Games left for greener pastures. Tilted Mill Entertainment is just a shadow of what Impressions Games once was.

I don't have a specific favorite, but on the criteria that if Full Immersion was possible, I've always wanted to stop into Myst Island.

I loved the Marathon series. Infinity ignited a passion for incomprehensible stories that drive people to argue over what really happened online.

Also, how many games would people say constitutes a 'series'? If I can get away with two entries I really enjoy the Survival (action) Horror series Evil Within. Unforgiving, weird, mysterious with a great atmosphere. Didn't like that they changed the voice actors for the sequel but really liked the characters.

Ignoring that most series I like have their stinkers, then:

Serious Sam (except SS2)

Age of Empires (1&2 are legendary, 3 was passable)

Command & Conquer (up to Red Alert 3, anything past that does not exist)

Legacy of Kain (Blood Omen 2 was pretty meh compared to others though)

Overlord (except the attrocity that is Fellowship of Evil)

Worms (especially Armageddon and World Party)

Tough question. To make it a little easier for me, I'm going to interpret it as "which ongoing series will make you want to buy and play each new installment" and extend it to five nominations, in that case it might be:

- Saints Row
- Far Cry
- Tomb Raider
- Life is Strange
- and everything with SteamWorld in the title

Wizardry

Huh? It's a PB "signature" (unfortunately) to tie class to faction. How do you learn spells (maybe past the very first) otherwise?

Quest for Glory! This will always be my favorite series that I love to return to. I've been playing this a lot when I was a child, so obviously there's a big feeling of nostalgia to it. But I also feel these series had a major impact on my imagination and further development of interests. It might sound funny, but I think I wouldn't have been the same today if I didn't have an opportunity to play it as a kid.

The other one would be Blair Witch series, with a strong emphasis on the first one. I still wish it was on gog.

The first game series that pops into my mind is Shin Megami Tensei, so many fantastic games there. I do enjoy Metroid as well, but as with most game series, there are 2 - 3 games I really enjoy and a couple of games I'm indifferent to.

Most games that develop into series tend to spawn tons of mediocre games.

I've got around 50 hours in Dark Souls Remastered. Not all in one character as I've restarted with multiple builds... but my love for it grows more and more. I strongly suspect this series is going to become one of my favorites based on my experience so far. Very briefly started 2 & 3 and I really liked the looks of them as well.

Can’t remember if I played that one. I was indicating 1,2,3,4.

They're only available on a proper subset of PCs, not all of them. (Also, apparently the new version of the client is a bit of a resource hog, which further limits what can play them, particularly low-end machines and (likely) those with cooling issues.)

Problem with the "cinematic storytelling" is that it gets in the way of the gameplay.

Also, Final Fantasy 5 isn't that simple once you get a decent selection of jobs and abilities.

No, smart phones are not the same as PCs. The interface is different (you don't have either a (phyiscal) keyboard or a mouse as standard, whereas you *do* have a touchscreen (not standard on PCs)). Also, the OS on them handles certain things fundamentally differently (apps are more isolated from each other than separate programs on a PC, for example). In fact, I would say they differ from each other about as much as either platform differs from game consoles.

Assassin's Creed even for me: till Revelations, then i really lost interest 'cause i didn't like Desmond'd fate.
May I include the Myst saga? I played it till Myst IV and I really enjoyed all those point & click adventures. They gave to me another dimension to dream about...

To put it another way, I treat games only available on DRM-encumbered platforms as if they were only available for systems I don't own, like for a modern video game console or a Macintosh (there's no way of playing Macintosh games without Mac OS).