What? Blasphemers! :p

I loooved the original, as well as every other game Blizzard put out (Lost Vikings included) save for that one terrible expansion to the first Diablo that they've almost succeeded in wiping all memory and traces of from the internet lol

Anyway, I thought of another franchise that seems on course for a list like this, though there's only been three installments so far: Pikmin!

Pikmin 1, 2, and 3 were all great games and offer a unique experience that you just don't find in any other franchise. I think once the 4th installment rolls around it'll warrant a spot on a list like this.

Sly Cooper I guess. It's only had 4 entries but all 4 are great!

Uncharted 4. Final Fantasy had a great run until FFXIII.

Both have had relatively bad games though.  Cod Ghosts is not well loved and Cod Vita is definitely not well loved.  Madden in the old days were pretty bad.

Cod had to make a significant risk in order to become so popular.

Cod Ghosts isn't well loved, but it still follows OPs criteria of:

"What franchises have stayed strong throughout the years, and why? What long-running franchises have never turned in a bad game?"

CoD Ghosts, for all the flak it gets isn't a bad game. I'll give you Vita, but I consider that a spinoff. Madden I dunno, you're probably right.

The old Maddens have not aged well obviously, but at the time they were considered the best thing you were going to get for a football game.

Honestly I thought the early Blitz games were a lot more fun at the time. As time went on Blitz got worse and Madden got better.

At the risk of inviting a wave of incredulity and anger upon myself, I'd actually argue Uncharted has two sub-par outings: Drake's Deception and Golden Abyss.

Drake's Deception is a gorgeous game, but the shooting and moving mechanics are awkward, melee combat is clumsy, the final boss battle is a joke, the story seems recycled, and there are far too many sections where we as players are asked simply to press forward while some scripted segment unfolds - walking through the desert being the most egregious example. Drake's Fortune is excellent and Among Thieves is outstanding, but, to me, Drake's Deception was a misstep, narratively and mechanically. It feels like Naughty Dog focused on the negative parts of Uncharted - scripted platforming, cinematic pretensions - and moved away from the positive parts - improvisational gunplay, smooth and seamless moving and shooting mechanics.

As far as Golden Abyss goes, it's a modest game that's sunk by a weird and unnatural reliance on touchscreen controls - especially during the final boss fight.

Just about anything Valve or Blizzard

Half Life just to name one.

God of war, including ascension.
Uncharted, Halo (lost the appeal it once had but you can't say any of them have been bad)
Mario Kart, 3D Mario, I could go in for a while we're lucky to have to many great franchises.