Speedyfast here again, Could someone tell me the all of the rarest/legendary..

Mew: Sorry, no other way than a cheat cartridge!
Mewtwo: Can be caught in Cerulean Cave after fixing Celio's Network machine with the Ruby and Sapphire gem. Hard to catch: master ball recommended.
Zapdos: Can be caught in the power plant with Ultra balls or Great balls.
Articuno: Found in Seafoam Islands, possible to catch with a lucky Ultra ball.
Moltres: Found at the top of Mt. Ember. Kind of easy to catch but take at least 30 ultra balls, just in case.
Lugia: You can only get this by going to a Nintendo event and downloading the ticket to Navel Rock, aka Island eight. But, of course, you could just use a cheat cartridge.
Ho-oh: The same way as Lugia.
The three legendary dogs: You can catch one of them depending on the starter you chose: Bulbasaur = Entei, Squirtle = Raikou, Charmander = Suicune. Your legendary dog will be roaming the kanto mainland after you do the Pokemon league and fix the network machine.
Celebi: Can only be obtained with a cheat cartridge.
Kyogre: You can trade this from Pokemon Sapphire, it is the only way apart from using a cheat cartridge.
Groudon: Trade from Ruby.
Rayquaza: Trade from Pokemon Ruby or Sapphire.
Three Regis: Trade from Ruby and Sapphire.
Latias: Trade from Pokemon Sapphire.
Latios: Trade from Pokemon Ruby.
Jirachi: Cheat cartridge, download off Pokemon Channel or get off Pokemon Colloseum bonus disk.
Deoxys: Download the Aurora ticket to Birth Island, aka Island nine at a Nintendo event. Or, just use a cheat cartridge. Only level 30; shouldn't be too hard to catch.
Well, hope I helped!

Ok, the legendary Pokemon for SAPPHIRE ONLY are regice, regirock, registeel, kyogre, rayquaza, and latias. rare ones are chimecho (look for at top of mt. Pyre), surskit (found on route 102 during the right season; check the tv for "rare outbreaks of surskit"), ralts (look where wally caught his), and skitty (either trade or look on route 116, also check tv).

To get them: kyogre- it appears in your game in the cave of origin in sootopolis and you get one shot to capture it. If you make it faint, there goes your chance. Unless you start over from where you last saved. So save before approaching kyogre. It appears once before in seafloor cavern, but it escapes before you capture it. Also, DO NOT USE YOUR MASTERBALL ON IT!!! You need it for either latias or rayquaza. Bring lots of ultra, great, and timer balls.

3 regis- solve the mystery of the sealed chamber (i'm not going to explain how, sorry). Regice is located on route 105 (you have to surf to it), one at the very bottom of the desert, and one on route 120. Use either great balls, ultra balls, or timer balls. I'd prefer great balls. Cheaper, and it works!!

Latias- i'm afraid this one, you just have to wait for it to come to you... Look for it in grass or surf. You can only find it after you beat the elite four!!! Once you encounter it once, the rest is easy. If you want to do it my way, sure, but if you want to do it any other way, sure! I go to the weather institute, and go in, then out. Then I check on my pokedex to see the area it's in. If it's in my area, I get a repel or super repel and look for it. It comes to you really quickly if you use repel. Either throw a master ball at it or save it for rayquaza. You can catch it without the master ball. It will run away after one attack or one ball thrown at it, but it keeps the damage for the next time you run into it. If you have a Pokemon with mean look, put it in front and use it! If you have a trapinch with arena trap, also keep it in the front of your party. It takes patience! Use ultra balls!

Rayquaza- I have to go now, but i'll be back!!!! (muahahahahaha)

Okay, it's me again.

Rayquaza- you can only catch rayquaza after beating the elite four. Surf to sky pillar (which appears after you beat the elite four) and should be on your map. You HAVE to use a mach bike to get to the top of the tower, where rayquaza is. The "bike puzzle" is very difficult because you can't slow down at turns or the floor will break. Once you get to the top, SAVE!!!!! Then go to rayquaza. You'll need a lot of ultra balls, and timer balls. Or use the master ball if you still have it. Rayquaza is very difficult to capture since it is level 70 and it knows rest. Maybe some other person in the hints section posted a really good hint there on how to catch it and the other legendary pokemon. So take a look there!

PS. NONE of what I said in this post and the one before involves cheating :D.

Happy playing!

Kyogre-Sapphire
Groudon-Ruby
Latias-Sapphire
Latios-Ruby
Rayquaza-Ruby and Sapphire
Regirock-Ruby and Sapphire
Regice-Ruby and Sapphire
Registeel-Ruby and Sapphire
Other Pokemon have to be obtained by buying special things.
An easy way to catch them is by getting a premier ball and lower the Legendary's HP into the red zone.

If anybody needs to know an answer to a question, ask me!
From Ultra Cheater