I'm something of an irregular hack player but the VLDC compilation hacks are always great. Everyone except a couple odd people at the bottom puts their best foot forward, so 80%-90% of the levels are at least decently fun and almost anything in the top 50% will be a unique, exciting, and fun experience. Every level is playable from the start and is designed to be a complete experience on its own, so there's no pressure to beat levels you don't like or are too difficult.
VLDC 11
VLDC 12
Super Mario World Central Production 2 is an extremely high-quality hack. Some of the levels are rather long and difficult and use iffy gimmicks, but there's a huge amount of content and nearly every level is expertly crafted. It also has an amazing, all-original soundtrack that is worth listening to on its own. Fair warning, though, it's much harder than Standard: Normal; it's more like Hard or even Very Hard. I currently have over 1000 deaths and 40 hours of playtime and I still haven't beaten the last four levels of the special world.
Super Mario Universe is a pretty standard, but very fun hack. It's pretty average difficulty, but I enjoyed not dying ten or so times every level. Good music and each level has something new to offer.
Last but certainly not least,
Super Mario World - Mario's Expedition is a Metroidvania-style hack that is insanely enjoyable to play. Of all the hacks I've mentioned, this one is the most raw fun. Most of the levels and areas are pretty standard SMW ROMhack fare but there's just something about the way the world is structured that makes you keep coming back for more until you've found every last secret. Difficulty is pretty average for an SMW ROMhack, pretty similar to Super Mario Universe.