Here's a few other hacks to add to this thread, ones that I rarely see recommended, yet I feel are both great choices for introductory hacks.
The Hooray! Fishing Season and its
sequel are very fun, relatively easy hacks with quite a few clever ASM gimmicks that the creator coded themselves. Mandew has a touch for fairly easy, yet interesting level design. I think these are a good choice for beginners if you're looking for something traditional and forgiving, yet with a hint of the fancy gimmicks and wacky ideas present in many way harder hacks that people often recommend (like JUMP 1/2 and JUMP).
On the Japanese side of romhacking,
Super Mario : P : P : P is similar to the Fishing Season hacks in that it's smaller and more accessible than some of the huge difficult hacks people typically recommend, which I think makes it solid for beginners. It's full of extremely wild gimmicks and clever design. Unlike the often recommended Brutal Mario, which is more of an ASM gimmick showcase than anything (nothing wrong with that of course if you're into that), this hack features both clever ASM and level design. If you want to dip your toes into what the Japanese hacking scene has to offer, I think this is a good one to start with.
SMW Yeah is also a good choice in that regard, though it's much longer.
A hack I rarely ever see recommended (or even played) is
Super Mario Balloon World. I've worked on a lot of projects with the creator, so I may have some bias, but I think this hack is pure genius. It takes the original SMW and adds balloon challenges, turning each level into a puzzle or speedrun challenge. The hack gets an incredible amount of mileage out of such overtly simple premise. I think this is a solid introductory hack if you enjoy the original SMW, but want to play it with a completely new twist.
