Generation I and II had enough legendary Pokemon to call the legendaries "rare". Generation II was a bit of a disappointment as there wasn't an "uber" legendary for that generation; Lugia and Ho-Oh are too much like the mascot trios of later generations.
Generation III probably did the best as far as legendaries go. Though the "Regi" trio and "Lati@s" twins felt out of place, I did enjoy a bit more variety. I hope they can do something later with the Lati@s twins such as introducing a pre-evolution of some sort (don't freak out yet- pre-evolutions of legendaries should be a long-winded process to obtain, maybe something similar to getting access to battle Spiritomb?) The introduction of "themes" in this game and tying the theme in with the mascot Pokemon was brilliant. I don't view the trio as a "weather" trio, but as a trio representing land, sea, and sky. One little nitpick: I don't like how Rayquaza has 10 base stats higher that others in its trio.
Generation IV went overboard on legendaries. I really hated Generation IV when it was released because of the excess of legendaries. While the time/space theme was definitely the ultimate, it feels like Celebi lost its importance as a time-travel Pokemon now with Dialga as a time-guardian Pokemon. Arceus is really too uber and defeats the purpose of Mew. Mew still can learn more moves than Arceus, but not enough to compete with Arceus's heightened stats. That's my main bother with Arceus- it puts all Pokemon to shame in a bad way just for having 720 base stats. Even the other ubers, Mewtwo and all the formes of Deoxys, succumb to it. Deoxys has 600 total in base stats, so why can't it? Just... arrgh... so upsetting...
Another thing with Generation IV, there were one (or two on how you look at it) too many "cute" legendaries. It seems like the only "cute" legendary that actually had a purpose was Mew. Celebi, as stated earlier, lost her purpose to Palkia, and Jirachi is just kind-of "there". Okay, Shaymin is a nice treat with two formes, but seems a bit outplace like Manaphy and Phione. Manaphy and Phione don't even make any sense.
Generation IV also sprinkled a frivolous amount of legendaries into the mixture. I'm okay with Cresselia and Darkrai as with Latios and Latias, but the moon duo seems to have a better legendary balance. However, it feels like there now needs to be a "day" legendary and "night" legendary, and I don't think Lugia and Ho-Oh fit that description as much as the fandom wanted it to during the speculation period for a possible remake of Gold and Silver. The "Lake" trio just felt like three poorly-looking Mew creatures that have little difference among the three. They're all the same type! You'd think that maybe one should've been considered to be dark-typed and one to be ghost-typed (or light-typed if they ever introduce it). Regigigas I'm on the fence on. Seems like it should've been introduced with the other three Regis (which have just recently made sense why they are those types after watching that particular anime episode). I really dislike how in the PokeDex it says something about Regigigas pulling the continents together with its mighty arms. I'd say it's interfering with Groundon's job, and with the natural concept of continental drift. Also, it looks like it should be a grass type because of the grass growing on its shoulders and I doubt that it would be able to pull continents together. Just... no.
Heatran is by far the WTF legendary. It has no relation to anything in the game, other Pokemon, or even in the anime or other types of media. It's the only legendary to either be male or female. If anything, it looks like one of the legendaries that least needs a gender, or a gendered Pokemon that shouldn't be a legendary. Not only that, it's really just a byproduct of the creation of the world that has no real "legend" behind it. It's in the "other" section of legendary Pokemon on Bulbapedia! The Twelfth Movie only made less sense to why Heatran would be a legendary considering that it was a "captured" Pokemon. Legendaries, as far as anime rule goes, shouldn't be captured. It's acceptable with Brandon, because Regis are almost everywhere and he's a Frontier Brain but not for that emperor. It didn't even do much but walk around in that movie!
