So, in all forms of media there are those series that should just stop. They've been flagellated to the point that they're barely recognizeable when compared to their early iterations. There's Saw in the movies, spinoffs of Flava of Love in TV (or even spinoffs of its spinoffs) and, well, these five videogames. Presented in order of most dead horses beaten. (Note: Tab of "dead horses beaten" does not include phone games, or rereleases of previous versions for new tech)
Guitar Hero (8 games, debuted 2005): Harmonix was already milking the "yearly update plus spinoffs" bit when Red Octane took their ball to Activision, but Activision diddled this franchise to a whole new level. It's at the beginning of the list because it has the least iterations, but it's also by far the youngest series. Not even 4 full years out of the chute, and there're already 8 games in the series, 4 of which came out last year! And this year, Activision has plans for at least 4 more, with the first being the Metallica version coming at the end of this month.
Final Fantasy (39-ish games, debuted 1987 (in Japan), 1990 (in the US)): I'm all for the numbered entries in the series. They come at a rate of about 3 per generation (and there hasn't even been one yet this generation), and that's hunky dory by me. The spinoffs, pseudo-sequels, snowboarding games and remakes (My god the remakes!) are what drives me nuts. See, that's why the series has "39-ish" games. That doesn't include all the quick cash-in cell phone games or the remakes. So, that would take the total to something I'd need Doc Brown to calculate for me. I'm guessing a google-plex-ish.
I mean, I understand that the first game saved Square. That's fine, it's your cash cow. That doesn't excuse barely letting two months go by without releasing Final Fantasy: Some Guy with Bad Hair and Non-functioning Zippers on His Clothes Whines and Fights the Ultimate Evil Part XVI: Sometime Near the Crisis. Give it a rest, guys. Seriously.
Sonic (48 games, debuted 1991): Really, Sega? Haven't you figured it out yet? Every new "Sonic & his furry friends do stuff that isn't just running really fast and twitch platforming" game just takes more and more away from the legacy of a series that used to be competition to Mario. Give the poor blue woodland creature a break for a couple of years while you come up with something new for him to do. (PROTIP: Start with running really fast and twitch platforming.)
Mega Man (70 games, debuted 1987): Ok, I love the Blue Bomber as much as anyone, and I have the tattoo to prove it, but Mega Man is Capcom's most prolific star (and that's saying something!) and maybe 35 of those 70 games is worth playing. Now, last year's Mega Man 9 was, in a word, amazing, but that doesn't justify three versions of the mediocre Mega Man: Star Force, which was, itself, continuing the mediocre and over-exposed Battle Network series. One 2D Mega Man game every other year would be a wonderland, because I'm sure Capcom and series auteur Keiji Inafune would take the time and care to make it really, really good.
Sports games (NBA 2K, NBA Live, Tiger Woods, Madden, MLB The Show, MLB 2K, WWE Smackdown, etc.) (Too damn many, annual releases began in the 16-bit era): This has been a thorn in the side of many (gullible) gamers, myself included, since the 16-bit days, but, hey, everyone wanted to play with an updated roster. But even in the 16-bit days, we still usually got some gameplay innovation at the same time. Now, you basically pay sixty bucks for a roster update.
With DLC and every system on the market being hooked up to the intertubes, there's no excuse for this. Take some time EA/2K/THQ and make an innovative game. Use DLC and make me pay five bucks to update the roster once a year.