There are many ledendary pokemon, but you can catch almost all of them!
Registeel
Head to route 120 to find a cave, titled Ancient Tomb. Once inside, interact with braille; the riddle requires to position yourself at the direct center of the room and then use fly. If done correctly, the door to Registeel should open, allowing you to capture RegiSteel.
Regigigas
After capturing Regice, Regirock, and Registeel you can begin your journey to find Regigigas. He is located in the Island Cave on Route 105, where Regice was originally located(up above). THIS IS VERY IMPORTANT: Regigigas won't show until a day after you have captured the previous three Golem Pokemon and will only appear during the daytime. You will need Regirock, Registeel, and Regice in your party. Make sure Regice has a nickname and is holding a ice-related item. If you didn't nickname your Regice when you captured it, take it to the Name Rater in Slateport City. As for the ice-related item, there are three options are the Casteliacone (found in the Trick House), a Never-Melt Ice (found in Shoal Cave and is the one I used) or an Icy Rock (found at the Weather Institute) I don't know if the Icy Rock nor Casteliacone will work if someone did it with those two please tell me. Enter the back chamber of Island Cave on Route 105 and walk to the center to bring out Regigigas.
here are Regigigas' moves which are helpful.
Latios and Latias
Southern Island is a very small island that cannot be easily reached. The first time you visit will be with Steven on the back of either Latios or Latias. Go north to find the other legendary Eon Pokemon Latios (in Omega Ruby) or Latias (in Alpha Sapphire). Depending on your version, Team Aqua or Team Magma will come by and challenge both you and Steven to a Double-Battle blah, blah, blah. You destroy Team Aqua or Team Magma and Steven(if you hate him that much, he is pretty easy to beat).
Kyogre and Groudon