A few of the popular hacks are actually really good in my opinion, so you could try those, many of them were already nominated here (like SMB3 - The Secret Place and SMW2+3 - The Essence Star) and since some hacks I would've nominated here were already nominated by other posters here that aren't very popular, I'll go ahead and suggest some that weren't mentioned here yet.
The Senate
In my opinion, it's a really good hack. Good variety, high-quality level design and one of the best examples of hacks that didn't go wild on ASM, but only on variety and level design. I highly recommend this one, you'll have fun for sure!
Sicari
If you're looking foward for something very unique then you totally should give a shot on this hack. You'll get impressed with the amount of originality on graphics of this hack and with the effort the author put on the hack. Another highly recommended.
Super Mario World: The Secret of the 7 Golden Statues
Even though the level design on this hack isn't awesome, it's still pretty decent. But I can assure it's a very fun hack to give a shot on, since most of the new features make the hack more exciting to play and the hack also shows good variety as well.
Collection
This one is very beatiful. Aesthetics for all the levels are very lovely, high-quality level design and a hack that shows a really great example of variety but still keeping consistency in graphics. In a side note, you could search for more hacks made by the author, ZMann.
Something else v1.02
A masterpiece! You'll totally have fun with this one, it has a great level design in general, many unique levels and an interesting story. Totally recommended.
Bonni's Quest [Demo 2]
Another masterpiece. Level design is of high-quality, the graphics are lovely and aesthetics for all the levels are divinely great! It's a very fun hack and quite unique, you'll see when playing the hack by the different gameplay and design ideas the hack has in general.
Super Mario Bros. 3 X v2.5
I'm also a fan of traditional hacks, mainly those inspired on SMAS. This one is a really good example of traditionality, I suggest you to play this one as well when you get some time.
Well, I don't have more hacks to suggest for now; I'm kinda forgetful after all, therefore I'll update this list sometimes if I remember more hacks I liked a lot at the point to consider them favourites.