The title says it. Almost everytime Eminem pushed himself over the edge of a specific aspect of rap, it was everytime when he was featured on some other artist's song.
His greatest considered verse of all time - Best Friend - Yelawolf song.
Verse where he went absolute batshit crazy with speed, arguably his fastest - Speedom - TechN9ne song.
His Verse where he rhymes in the most technical, complex and condensed way - No Favours - Big Sean song.
Arguably his most lyrically advanced song on every aspect: Rhymes, flows, Wordplays, Speed - Animals (Pt. I) - JID song.
The only aspect that I was not able to find him outperforming himself on some other artist's song is Storytelling, which imo he did his best in Legacy.
Is there a specific reason for this, if there is any? He put his best performance on features.
If I've missed anything, please let me know.