I haven't played many hacks, but I'll recommend these
- Pokémon Liquid Crystal: It's a very, very promising hack, from what I've seen so far [Though I haven't had much time to play the beta]
- Pokémon Shiny Gold: An awesome hack with lots of content, and very fun to play. If you're going on a long journey, I suggest you download Shiny Gold X, for the extra challenge.
- Pokémon rijonAdventures: Awesome Hack, I've not played very far into it yet, but it's an awesome hack.
- Pokémon Brown: A hack of the original red, it's pretty fun to play, aswell.