Melee's hit sounds are infinitely better. It felt great to hear Falco's Melee shine in the more recent P:M videos. Also, electricity sounds in Brawl bother the hell out of me. They were more subtle in Melee and heavy electric hits had that post-shock sizzle.
And I love Falcon's B-air hit. It's the same as a thrown HR Bat hit which sounds awesome. The Knee uses the hard hit sound and that electric sizzle.
Heck, I'd sooner prioritize replacing Brawl's hit sounds over Brawl's character voices. Less work, no need for frame data (for the most part), and the results are instantly noticeable.
But that's only if someone has the time to do it in the first place.