Command and Conquer (technically it is finished but the series didn't deserve to die with such an abomination like C&C 4)
CNC was so good until that shitstain game came along
Skyrim was pretty good, but ES Online is awful, and it has denigrated the series
COD has always been shitty (Modern Warfare excepted)
Apparently Battlefield Hardline killed it for many people
I though Halo Reach was the nail in the coffin for Halo, but they pulled it from the brink
Freelancer was such a good game, but they cancelled the sequel to focus on their shitty Xbox
I love that game today, and it came out in 2003.
Space Marine was exceptional, and brilliant, but the company went bust and they cancelled the sequel.
Dawn Of War was an awesome game, but DOW 2 was an literally an RPG, not a strategy game, and DOW 3 isn't coming soon.