I first played Altered Beast in the arcade when I was a kid, and it was decidedly cool but not that deep of a game really so it wasn't exciting as a Genesis pack-in to me. My older cousin got the Genny when it first came out, and got a bunch of the release (or close to release) titles that I remember so much more fondly like Thunderforce 2, Last Battle (I might be the only one who likes this game), Ghouls n' Ghosts, and Space Harrier II. (Admittedly not as good as I had hoped being a huge Space Harrier fan at the time)
Given the choice between Super Mario World, Keith Courage, and Altered Beast, I think Keith was still the best pack-in out of the 3 for me. KC had the anime vibe (and mecha vibe) I was completely into at the time, it also had the underdog appeal and was the least common and most unique among my friends, and that mentality has always worked for me. (I also liked the music and underworld stages a ton, it wasn't all elitist appeal, I honestly liked the game) I think the last cool Mario platformer to me was SMB3, I just always felt SMW is where everything got washed out and since then, Mario games have just been getting more and more infantile.