Wow, this is a pretty hard vote, there are some amazing hacks that are eligible for voting... I'd have to go with Pokémon Mitic Island! It is terribly difficult because my favorites are Mitic Island, Thunder Emblem, Zephyr, Omega and Topaz/Garnet!
The reason I'm voting for it is that Lex is one of my greatest friends, and his game's palettes and tiles are amazing! I mean, he's inserted as much tiles as I can imagine... So many different trees, maps, and other things that have amazed me. He's inserted like a million palettes for just a single tree, and I also respect him for all of his other hacks. So, Mitic Island is my vote!