Groudon and Kyogre's epicbattle in Pokémon Emerald. And more importantly, Rayquaza, who flies to Sootopolis after you encounter it at the Sky Pillar. Once you arrive there and witness Groudon and Kyogre again, Rayquaza comes out, flies through the clouds, descends with a trail of green light cloaking it, and swoops down to Groudon and Kyogre. With one mighty roar, it sends them both away, and departs, returning the weather to normal. How intense is that?
Earning the Gold Shield ornament after winning 100 straight battles in the Battle Tower definitely qualifies as one for the player and his/her Pokémon.
Or managing to get the gold symbols in the Battle Frontier, especially with all the crap they put you through.
Omega Ruby and Alpha Sapphire
In the original games, Groudon and Kyogre's special abilities automatically summoned permanent sun or rain to the battlefield. Unfortunately, a Nerf in Gen VI changed the abilities to just an automatic Sunny Day or Rain Dance. Then comes Omega Ruby and Alpha Sapphire, and the introduction of Groudon and Kyogre's Primal forms, who have two new abilities: Desolate Land and Primordial Sea which besides sounding really badass, came with a few new effects:
Made the sun/rain permanent again as long as Groudon or Kyogre are present.
Once one of the abilities is activated, the weather cannot be changed by a normal Rain Dance or Sunny Day.
And most importantly, completely negates water (if Desolate Land is in effect) or fire (if Primordial Sea is in effect) type attacks used against Groudon or Kyogre. Thought you could one-shot Primal Groudon with a water type attack thanks to it's 4x weakness? NOPE.