Can anyone please tell me what are the best pokemon for beating the elighe..

A fire/flying would be good against the whole elite four

for example

Charizard
dragonclaw(for dragons)
flame thrower
earthquake(for every poke on the ground)
any move of your choice

fighting/grass/electric(especally!) for the water/Ice

A water flying would be great against bruno
gyarados for example
surf
dragon dance
aerial ace
earthquake/any attack

any other's would be psychic/ghost for bruno

psychic/dark/ghost for agatha

dragon or ice for lance

Then you should be able to beat your rival with that type of team.

Try using a mixed team, like one of each type. There is no best pokemon really.

I would say...
Jynx level 77(trade Poliwhirl in Cerulean City)
Raichu level 74 (Catch Pikachu in Viridian Forest)
Lapras level 73 (Get in Silph Co)
Venusaur level 79 (Choose as Starter)
Dodrio level 74 (Catch in route just above Cycling Road near Celadon City)
Dragonite level 78(Catch Dratini or Dragonair on a rod in Safari Zone)
These are the pokemon i used and they worked just fine against the Elite 4.
Hope this works for you.

The pokemon I used were
Charizard lv 100 (starter from other file)
Snorlax lv 72 (blocking bike path)
Pigeot lv 61 (evolved from pigeotto)
Mewtwo lv 100 (from other file)
Blastoise lv 73 (starter)