My 50th game was the second to last game developed by a now disbanded developer.

@ Kantor

Metal Gear Solid? Prototype? Condemned? Syphon Filter?

Skyward Sword didn't make it, sorry Nintendo

That is quit easy actually, I've been planning on picking up this game for some time now. The demo was hell of a lot of fun.

My 50th game is from the last decade but could also belong in the 1980's.

Kantor, nothing freaking fits your criteria.

* is a famous series
* is on a decline
* has an earlier entry that will be on many member's lists
* has only one main competitor which is more popular and more realistic.

Only sports or car games come close but not them either. I'd say Need for Speed but that hasn't got a more popular and realistic competitor.

I'd say PES but someone mentioned it already.

Forza or Gran Turismo?

I didn't even particularly love the first two games (Pacific Rift was okay), but Apocalypse was just something else.  Crazy fun driving through a city as it crumbles.

I'll have to work on a more obscure clue for my next one :) 

No, you said it already but Kantor was silent like a mouse.

Plus I wouldn't think Kantor would claim that Tony Hawk is "a very famous series, which was once great and which has now gone to crap."
&
"one particular (earlier) entry in the series will feature heavily in these lists".

I agree, especially while talking about games like REmake and FFIV on DS

Metal Gear