The first elite four member is Will. He specializes in the psychic type and has two Xatu, a Jynx, an Exeggutor, and a Slowbro.
The second elite four member is Koga. He specializes in the poison type and has Airados, a Venomoth, Forretress, a Muk, and a Crobat.
The third elite four member is Bruno. He specializes in the fighting type and has a Hitmontop, a Hitmonlee, a Hitmonchan, an Onix and a Machamp.
The fourth and last member of the elite four is Karen. She specializes in the dark type and has an Umbreon, a Vileplume, a Gengar, a Murkrow, and a Houndoom.
Finally, the champion, Lance, specializes in the dragon type and has three Dragonites, a Charizard, an Aerodactyl, and a Gyarados.
I think you should take:
Noctowl (Its flying moves will be super-effective against most of Bruno's team. Just watch out for the Onix! It's also a psychic type and can learn psychic type moves that will be super-effective against Koga's team and a few of Karen's Pokemon)
Gyarados (This is a really good Pokemon to bring to the elite four. Not only is it very strong, it can learn and be taught by TM some dragon type moves, which can knock out Lance's Dragonites. Gyarados will also be useful for beating Bruno, since it is a flying type)
Dragonair (You need all the help you can get against Lance's Dragonites, even if that means risking Dragonair being knocked out by their dragon type moves)
Ampharos (Ampharos is a good Pokemon that can knock out Will's Xatus, Koga's Crobat, Karen's Murkrow, and Lance's Gyarados and Charizard with it's electric type moves)
Ho-Oh (For one, Ho-Oh is a legandary, so it is naturally stronger than most Pokemon. Also, with it's flying and fire typing, it will be good against most of Bruno's team, Will's Exeggutor and Jynx, Koga's Ariados, Venomoth, and Forretress, and Karen's Vileplume)
Feraligatr (This was your starter Pokemon, so I think you should stick with it throughout the game. However, if you don't like the idea of having two water type Pokemon on your team, you should replace it with Piloswine)
I hope this helped :)