I have enjoyed two Pokemon themed hacks of Super Mario World.
Super Pika Land Ultra and Pokemon Darkness.
In Super Pika Land Ultra, a lone Pikachu wanders into the Mario universe for unknown reasons. (a shout-out to a GB hack of Super Mario Land titled Super Pika Land)
The first stage is called "Beginner Greens" which sounds like an easy level, but instead it turns out to be hard. This game is really difficult, levels ranging from plain hard to Nintendo Hard (especially the volcano level which is insane).
In Pokemon Darkness, you play as Mew, who has awakened after sleeping for 300 years. And finds out that Mewtwo is causing havoc to the Pokemon world.
Unlike most Super Mario World hacks, this game uses a Metroid style life system. Unfortunately you won't be able to get more life than 99 (i.e. no energy tanks).
Collect as many coins as you can so you can buy potions, TMs, and Rare Candies.
Unlike in the RPGs it raises levels, it makes you invincible for a short time.
Defeat bosses to get stones to progress into the game.
The hack however is in French, but an English translation of the hack is being worked on.
The hacks can be found at
http://www.smwcentral.net
I would like to see these TASed, especially 100% runs.