Usually every certain years, Super Sentai would pull off Anniversary show and they have been rather different for years. Let's see how it evolved.
The first special is the Turboranger special episode which began even before Turboranger's first episode aired. It honors the Super Sentai team starting from Battle Fever J until Liveman (at that time, Goranger and JAKQ hasn't been considered) and mostly just does some clip shows of those past Super Sentai and it ends with those teams cheering for the Turborangers to fight the Bouma and carry on their legacy.
The second special is the Super Sentai World which heavily featured the 16th series Kakuranger. Rather than a clip show, it calls in the Sentai warriors from Fiveman, Jetman (Gai is probably not dead yet here), Zyuranger (back from the heavens) and Dairanger (not yet old folks) in order to fight an original enemy. While this probably used the crossover nature a lot, I think it was the beginning of it. Most the pre-Kakuranger warriors don't appear out of suit.
During the Ohranger's airing, probably due to downed ratings, they may be thinking 'Only an Anniversary show can save us now!' Thus they drafted Goranger and JAKQ into the roster and after Ohranger is done, they released Carranger, an Anniversary show which is... a parody of the Sentai genre. They don't do clip shows or crossovers, they just... run along with the show, poke fun of the genre. And it saved Sentai too to continue on.
Now the next anniversary is Gaoranger vs Super Sentai. In a way, this carries on similar elements with the Turboranger special, there were clip shows, but in it, also a new original story, and all warriors from Past Sentai also appear to give their approval for the Gaoranger to continue on their legacy (in this case the whole Red warriors). But what makes this unique is that this also gives us a crossover Sentai team, a Sentai team whose members are from different Sentai teams (in this case Big One, Red Falcon, Ginga Blue, Mega Pink and Go Yellow). I think this is where the awesome really go.
The next anniversary is Boukenger vs Super Sentai. This is really similar with the Gaoranger one, except they don't use whole clip shows and limits it to the previous shows that appears after the last anniversary (Hurricaneger to Magiranger), similar to Super Sentai World. It also introduces us... Aka Red, who'll later appear in Gokaiger. A good showing, but felt like it could be expanded further. I mean, the concept on Gaoranger vs Super Sentai is awesome, a practically crossovers that knows no era or decade, Boukenger vs Super Sentai, while good, felt like a dumbed down version. Although there are anniversary element in the Boukenger show itself, starting from fight coreography into how every single enemies are based on past Sentai robos.
Then there's Gokaiger, which also takes note from Carranger: it's a show that runs on its own and is filled with nostalgia factor and cameo from past rangers. No clip shows, no crossover teams, just an awesome story filled with shout outs to the past.
I hope you had a good read about this.
Also for bonus stuffs you can do, if you are aware of my fic, you realize I am making up my own crossover on Super Sentai which is heavily based on Gaoranger vs Super Sentai. So I'm wondering, if you guys who read here would like to make up crossover teams, which would be your team? Minimum 5 characters, maximum 7 characters! Include villains too if you can! You can put them in your comments! You can see mine in these posts of my blog: