Godzilla
Of course, the most hardcore fans would argue that Godzilla nuked the fridge after the first film, others say Ghidorah the Three-Headed Monster (because Godzilla stopped being a villain), etc. As a fan of the first series of films, Godzilla (Toho; 1984), the first film in the second series, was where the franchise nuked the fridge, IMHO. The SPFX are among the best ever in the series, but God, the story was a mixed bag of epic proportions. They tried to return Godzilla to being a villain, but he comes off being more like the original Gamera than the atomic demon from 1954! (He only crushes cities because he's clumsy.) Not to mention that the character of Dr. Hayashida (Yousuke Natsuki) was a grown-up version of Toshio Sakurai (the kid from the first Gamera film). The rest of the films after that, especially in the third (and last) film series, are hit-and-miss.
Gamera
This is difficult to determine where it nuked the fridge, because I actually enjoyed all of the Gamera films (except for Gamera Vs. Barugon, which was WAAAY too dark for my tastes; with the exception of that film, all other films are more consistent with each other). I also enjoyed Shuusuke Kaneko's Heisei Gamera Trilogy (although G2 was my least favorite of the three; the other two are great)! I've only seen bits of Gamera the Brave to date, though I was impressed with what I saw.
Ultra Series
If you want to be really anal about it, some fans thought Return of Ultraman, Episode 38 ("When the Ultra Star Shines"), was where that series jumped the shark (though one could argue that Return of Ultraman was where it jumped the shark in general for tampering with the timelines of the previous series!). But personally, I'd say that Ultraman Ace was where it jumped the shark. I mean, don't get me wrong, it's got some great moments (Jared will go on and on about it!), but compared to the previous shows, this is the point where the Ultra Series becomes more like Hanna-Barbera's Superfriends shows. BUT, it's faaaaar more entertaining and exciting than the Superfriends shows!
Masked Rider Series
For me, it jumped the shark with Masked Rider Black RX. And it never got better after that...
Super Sentai Series
Ninja Task Force Kakuranger. Nuff' said.