There is no "best" team. Mostly its personal preferenace, what you think is cool, cute, strong, looks awesome etc.
I can suggest a team though. Off the top of my head, try this :
1. Samurott (Oshawott's final evolution) - Water Type
Try and stick mainly to special attack moves (purple swurl symbol)
2. Simisear (Evolves from pansage) - Fire Type
** note : once its evolved it won't learn any other attacks from leveling. Tm only.
Mixed attacker, he can used Physical and Special attack moves
3. Swanna (Evolves from Ducklett) - Water and Flying type
**note = Even though you already have a water type, water/flying is a great combination as it adds no extra weaknesses and actually reduces total weaknesses by 1.
Mixed Attacker
4. Elelktross (Evolves from Tynamo) - Electric Type
**note : it has the levitate ability so it has "no" super effective weaknesses
Physical Attacker
5. Wall Pokemon, choices are;
Scrafty (Evolves from scraggy) - Fighting and Dark
Escavalier (trickey evolution process envolving Shelment in party with Karrablast and trading it) - Bug and Steel
Cofagrigus (Evolves from Yamask) - Ghost
Scrafty and Escavalier are Physical Attackers. Cofagrigus is Special Attack
6. Depending on what 5th pokemon you took :
Seismitoad or Gothitelle / Reuniclus if you picked Scrafty
Semismitoad or Liepard or Gothitelle / Reuniclus if you picked Escavalier
Semismitoad or Liepard if you picked Cofagrigus
Gothitelle (Evolves from gothita) [Black Version] - Psychic
Reuniclus (Evolves from solosis) [White Version] - Psychic
Special Attacker
Lipard (Evolves from Purrloin) – Dark
Mixed Attacker
Seismitoad (Evolves from Tympole) - Water / Ground
Mixed Attacker
note** = I know if you take this pokemon you'll effectively have 3 water types but seismitoad is only weak against 1 type : Grass.
It can also learn a great array of attacks : ground, ice, water, rock, fighting, poison.
Hope you like or find oyur own team. Enjoy the game :)