I'd say it's a tie between sun and rain.
Sun, on one hand, gives boost to fire- and grass-type hits, if I'm not wrong, and weakens strength of water attacks. Sun is easily summoned with Sunny Dance, and would become even more easy if we happen to have some Pokémon on our team that has Drought ability (good example of this would be Ninetales). Rain, similarly, is not very difficult to take out, either; it could be done in an instant. When it's raining, we get power raise in water-type moves.
I've utilized both weathers in strategies before while battling, and they seemed to have worked flawlessly; am definitely a big fan of them. Aside from these two, I'd say one of my other favorites is hail.