There is really no best team to beat the E4 with. However, these are some Pokemon who might help make things easier for you:
Magneton: Its Electric typing helps take down a good number of Lorelie's team, and some on your Rival's team. Steel gives it a great number of resistances.
Exeggutor: His Grass typing deals with some of Bruno's Pokemon, while the Psychic part deals with the rest. Psychic easily takes down Agatha. Once again, your Rival has some Pokemon that this guy can be used for.
Gengar: Takes down some of Lorelie's team, is immune to Fighting, good backup against Agatha, and some Pokemon on your Rival's team.
Dragonite: This guy gives extra backup to your Blastoise (who should have Ice Beam) when facing Lance. His wide range of moves helps him take down all of the E4.
Arcanine: Rounds things off with Fire typing, helping to take down your Rival's Venusaur, getting neutral damage a lot, and Jynx is no match for this guy.
Blastoise: If you want this guy, then sure, have him.