Ok, surprised it hasn't made it on here yet but I can name 2 game series absoloutely killed by their sequels, in fact they haven't ever been heard of again and ofcourse, anyone who claims a game series is done with has never played FFVII or seen the plethora of crap titles to follow it on it's name 10 years down the line.

Grandia was a charming if deeply flawed ps1 rpg, it didn't have the best storyline and the graphics whilst all polygon rendered were let's face it horrible even at the time. However it was a very enjoyable game to play it flowed with just enough speed and story at once to keep the player going through it's 2cd arc. Grandia 2 and subsequently Grandia 3 were amazingly horrible. I played 2 for all of 10 mins on the ps2 and whilst the graphics were a huge improvement from the first, they were still sub-par with similar titles out there and face it the voice acting...I wanted to go stab the voice actors in the face it's storyline also sucked hard. I never touched 3 with a 25ft bargepole.

And for the second entry on the list: Jade Cocoon, the first was if repetitive and complex a brilliant game. I remember now enjoying myself with the demo when it was first released for hours combining all kinds of creations. Jade Cocoon 2 was a complete pile of crap, gone was the human vs monster combat, if your monsters died you died...pussy character learn to fight with a knife like your predecessor! Secondly, oh dear god, the voice acting....I won't even finish the sentence just watch some on youtube if you don't believe how bad it was. Thirdly, Ka-hu yeah Ka-hu thats the name of the character, I believe it's written on screen Kahu but it's pronounced Ka-hu dear god, thats so bad a name for a protagonist I already feel the loss of faith. Fourthly, there was little to no storyline in this one. I could go on but face facts, too much digimon/pokemon not enough jade cocoon in the sequel.