SSX has gone downhill from day one

Of those you listed, definitely Call of Duty.

The first three were good, especially the first two. Call of Duty (1) will probably remain as my number two FPS for a very long time. It was very awesome. The series died on me when they started to focus on modern day warfare, which is not interesting in the slightest. After that it's gotten worse, when gameplay didn't change anymore and now are just yearly cash-ins.

I think most people mention Resident Evil because of the latest entry and it's abysmal ratings. Sure, it's less survival-horrory than it used to be in the beginning, but the games still know where they're going for now. Never played a Final Fantasy which is also a popular one to mention, they weren't released in PAL regions on the early Nintendo's so I never gotten into them.

Firstly, just a few comments about the games in the OP:

Command & Conquer

I don't really agree with this, but I see where you're coming from.  There's a definite difference between Westwood developed games and EA developed games.  That said, Generals was made fully under the EA label - and like you say, it was great.  Similarly, Tiberium Wars & Red Alert 3 are both really, really enjoyable RTS games.  I think people just gloss over them because they like to think that C&C has gone downhill, but actually if you play them they're a lot of fun.  RA3 keeps that campy sillyness in tact from previous Red Alert games and TW is a functional follow-up to Tiberian Sun.  The developments since then - with the free to play bullshit - is worrying, though.

Final Fantasy

I don't really agree with this either: you're right that the quality has gone down hill in a sense, but the quality is definitely still there, just dropped.  Besides, it's such a huge point of debate - for example, you say games after X was where it went bad, I'd say that X was the game that started making things bad.  I seriously don't understand people who praise X and then slate XIII - X was far more linear and had a far more stupid story.  Regardless, there has still been plenty of fun to have under the 'Final Fantasy' name since X which cannot be said for some other series out there.

My picks:

Spyro the Dragon

My avatar, my favourite videogame series growing up.  Fantastic collection platformers.  Then, it all started dropping off.  A new developer took the reins, tried to make a game in the style of the first 3, but rushed it and made a frankly broken product at launch (Enter the Dragonfly).  Then, a new developer had a go and made a 'decent' game that worked, but just felt nothing like Spyro (A Hero's Tail).  Then, we got the 'Legend' series, which was Spyro in name only.  Now we've got Skylanders.  While I hear Skylanders is actually a decent product, literally any name could've been slapped onto it, it's just not Spyro.

Crash Bandicoot

Same deal as Spyro, really.  New developer has a go, but makes a slow and buggy product (Wrath of Cortex).  Another sequel gets made that's actually an improvement - I don't have much to fault about Twinsanity actually.  But then we got the Titans games, which were fine, but they just weren't Crash Bandicoot.  Like with Spyro, I feel like they could've just been any series.

SSX

Same sentiment I've seen a few other people mention actually.  It's a hard one to explain.  The first 3 SSX games were fantastic arcade snowboarding games.  Neon-lit, challenging but fun, beautiful and entertaining.  Then On Tour appeared, which was a game I just never got.  After the open-world of SSX 3, why were we back to level-based challenges?  What was this Napoleon-Dynamite design?  Then we had Blur which was fine, but it was basically just SSX 3 with motion controls and no open mountain.  Finally, we had the new SSX, objectively a very good game, but just not an SSX game.  Everything that made SSX good seemed changed for this new 'dangerous snowboarding' design.

There's a number of series I also thought were starting to go downhill, but they ended up recapturing that magic with later installments and saved themselves.  Things like WipEout (Fusion was bad, but then from Pure onwards they were good again), Twisted Metal (3 & 4 were awful, but every one from Black onwards was great), Syphon Filter (3 was looking like the end of the series, but then Dark Mirror revived it in spectacular fashion) and Animal Crossing (I'm still divided on this one.  I loved the original, 'liked' the DS version and sort of enjoyed the Wii version.  They need to shake things up with it for the 3DS version, imo).

I was wondering how long it took before someone made that joke.

I kind of disagree with the reason Final Fantasy has gone downhill. It's not because the games are being made by "SquareEnix" instead of "Squaresoft" because both are basically the same.

The problem is, and you mention it, that they always try to improve and enhance the game/experience. You mention this for FF VII, FF VIII, FF IX and FF X as good things. And that's exactly what they did with the later games, only this time they messed it up.

And this is not because of Enix, who kept Dragon Quest been the same game until Dragon Quest X. Why would they change one franchise so much while keeping the other so unchanged? Because "Square" still makes FF while "Enix"takes care of DQ.

Square with its attempt to make it more appealing to the occidental market is the only one to blame for FF, but this is a sin that many Japanese devs share.

Anyway, for me it has to be Sonic. Yes, the latter games are getting better but it's still nowhere near the quality of the 16-bit era games.

I voted for Silent Hill, because it was a great franchise with healthy sales and popularity.
Since SH3, the franchise went downhill and each iteration failed to appeal to both old fans and new ones. Even tough I played Homecoming and (to me) was great, but I know a lot of people who didn't like it.
So yeah, that series is almost dead, take a look at Downpour and its sales.

2 was awesome and funner I think. My favorite of the two.  3 is the one that seems like it is really watering down the horror and difficulty

The only one on the list I think has taken a nose dive is Final Fantasy. Assassin's Creed got better for the most part. I prefer the Resident evils since 4 and including 5 for it's awesome co-op. Fable 3 was better than 2. CoD got better, but now is exactly the same and Silent Hill was boring from the beginning.

I just played the HD collection was bored as hell. It was slow, ugly, with terrible combat and most of all not scary.

Key Word for me is: Korny