Madden. Easily. COD for runner-up.

Yeah, Sonic for me. I'll go one step further and say that Sonic has never been good.

Madden, COD.

Another one is Assassin’s Creed. For a long time it was as annialized as Cawladoody and Madden.

Yeah this is probably my vote too, while they're not all bad, there's just so many awful games in the series that feel completely ill conceived. 

Pokemon.

It was a revolution in the GameBoy days, but it's been at least a decade of uninteresting, unpolishment and lack of creativity. Wind / Waves could change that for the better, but I'm not holding my breath.

I hate to say Final Fantasy...but the franchise's best years are definitely behind it.

Superman

Superman is really cool, so people made a bunch of video games starring him. To my knowledge, there were about 15 notable Superman games from 1979 to 2006, more if you count radically different versions of the same title on different platforms. And regardless of who developed these games, they basically all sucked. Sure, everyone knows Superman 64 was the worst thing ever, but it doesn't end there. The Atari 2600 game? Often considered one of the worst on the console. The NES game? GameFAQs ranks it in the bottom 10% of the NES library. The Genesis game? GameFAQs lists it as worse than the Genesis Bubsy games. The Death and Return of Superman? Got reviews of around 5 out of 10. Superman 64... Superman: Man of Steel for the Xbox? The most positive review says it "isn't a game for everyone, and the repetitive mission structure will more than likely deter all but the Superman faithful." Superman Returns? The final boss is literally a bunch of hot air.

 The best-received Superman game of all time was Superman: Shadow of Apokolips from 2002. It has a Metascore of 64.

I don't think I'm qualified to answer this because only hardcore masochists have sufficient experience in series that they hate. I think you'll need a lot of experience before judging a long running series as "the worst".

Some notable series that I did try playing a few games from and figured they weren't for me include Mario, Kirby, Sly, Ratchet, God of War, Sonic (ultimately), Call of Duty, Gears, Kingdom Hearts, Dynasty Warriors, most licensed titles, and sports games in general (liked some but eventually I got so bored and deleted the genre from my brain). But my limited experience doesn't qualify me to label any of them the worst. Among them I guess Dynasty Warriors is my least favorite gameplay wise, but if I paid attention to the story and played enough entries, my opinion would probaby change favorably.

Do we really need a Madden/FIFA every year?