The last hack I played was Emerald Genesis. It's got a few new areas to explore, a bunch of mons from gens 4-7, increased difficulty, and tweaked movesets and stats to make previously shitty mons viable (Absol in particular gets a big boost). I definitely recommend it if you're looking to spice up an Emerald playthrough without straying too far from vanilla.

What should I play next? My preference would be something similar to Genesis that just increases difficulty, tweaks stats, and add some new content on top of the vanilla game. I'm interested in gen 1 or 2 hacks as well.

What's the best hack you've ever played?