Initially I thought the "Rider War" from Kamen Rider Decade was spectacular enough.
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And then came the "Lengendary Great War" of Kaizoku Sentai Goukaiger =O
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Seems that Decade was so successful that Super Sentai is koping it's idea.
Both are anniversary seasons, both are the very first scenes of their respective shows, and both scenes featured a "war" of survival involving all the past warriors. Furthermore, both Decade and the Goukaigers have the ability to transform into previous warriors. But there are some differences as well. Let's see how these 2 compare.
History
Decade: 10th season of the Heisei Kamen Rider series (hence it's name)
Goukaiger: 35th season of Super Sentai series
Numbers
Decade: Each season had a different number of Riders, but the mass-produced Riotroopers made up the number...
Goukaiger: Each season has an average of 5 warriors, a few older ones had 3, the newer ones all had 6 or more. They can't even fit into the screen lol
Backstory
Decade: Riders from the previous 9 seasons team up again Decade himself, who's fated to destroy all other Riders
Goukaiger: Warriors from the previous 34 seasons team up against the Zangyack Empire. The Goukaigers themselves never even appeared
Other elements
Decade: Lots of other elements made appearances, among them (those that I can identify) Side Basshar, Castle Doran, DenLiner, ZeroLiner
Goukaiger: Why didn't their mecha appear at all D:
Outcome
Decade: Decade pwned them all, which forms the onset of Decade's destiny
Goukaiger: All the Sentai warriors sacrifised their powers to destroy (no they didn't) the Zangyack troops
So I guess the Rider War seemed to fare better from my point of view, but then the Legendary Great War (they should really come up with a less cheesy name) had a greater sense of familiarity since I could identify more familiar "faces".
But this is just a comparison of the opening scenes. Kaizoku Sentai Goukaiger has just started, and it's too early to give an overall judgement.