What is the best team to beat the Elite 4 in Pokemon Emerald?

One option is that you will need Pokemon that will be super-effecive against the Pokemon your battling such as Fighting-type Pokemon or Bug-type Pokemon which they would be super effective against Sidney's Dark-types, Ghost or Dark-type Pokemon since they're super effective against Phoebe's Ghosts, Fire-types for Glacia's Ice-types but also Electric-types for the ones that are part Water and Dragon or Ice-types for Drake's Dragon-types and level-wise they should be over Level 55.

Another option is to use a specific team set up for the Elite 4 such as for example Sceptile, Blaziken, Marshtomp, Rayquaza, Groudon, Kyogue. Other Pokemon that would be good against the Elite 4 would be a Level 56 Gardevoir, a Level 61 Blaziken, a Level 53 Regice and Registeel would be a good option as well.

Another option would be to use a team of Blaziken, Manectric, Starmie, Salamence and Rayquaza. Blaziken should have the moves of Bulk Up or SWords Dance as well as Overheat, Rock Slide and either Sky Uppercut or Brick Break, Manectric should have the moves of Charge, Crunch, Thunder and Thunderbolt, Starmie should have the moves of either Cosmic Power or Thunderbolt as well as Ice Beam, Psychic and Surf, Salamence should have the moves of Aerial Ace or Dragon Claw as well as Dragon Dance, Earthquake and Rock Slide.