I'm struggling to think of rappers who shine on others songs but are average in skill. I'm immediately coming back to rappers with great guest spots and who are just great in general, but have average/below average albums.

Inspectah Deck specifically fits in that category. He is by no means average. He has some of the best verses in Wu's discography, he's just never been able to replicate that level of dopeness on his own solo shit, though a flood is part to blame for that. The Czarface shit is pretty repetitive by the second album too.

This thread would be easier if it were "Name MC's whose guest spots overshadow their discographies" or something like that.