What *should have* been a favorite mid card feud for any child of the 80s was the Shawn Michaels/Marty Jannetty fued of (I believe) 1991. I still remember (vividly) watching Marty return and accidentally hit Sherry with the mirror (funny story, I was watching with my older brothers [i was probably 7 or 8 at the time] and my mother happened to be in the room and, shocked, she exclaimed "did he just hit a woman!?"...funny how we remember those things).
We were so pumped to see Marty return, and I remember my brother was rooting for HBK and I was rooting for Jannetty (which, perhaps, made the feud feel even more special as those were our respective "favorites" from their Rocker days). However, as we all know, for whatever reason ( I think I've heard Jannetty had his personal issues), not much became of what should've been a huge feud, sans the IC belt being exchanged a few times in forgettable matches. What a let down.
I also really enjoyed Brian pillman/Goldust (over Marlena) from the summer of '97...really seemed personal, and very risqué at the time (you'd never see that today, and I'm pretty sure they were still PG then). I'm assuming Pillman's untimely death killed (wow, poor word choice) what could've lasted and bloomed into one of the great all time mid card feuds. Pillman, Dustin, and Marlena all played their parts perfectly, IMHO.
Does SCSA/Bret Hart from late 96 thru summer 97 count ( maybe, technically upper/mid card)??
If not, I'd agree with the above posts that the Nation/DX feuds through 98 will likely go down as one of, if not the best mid card feud of all time (or the 90s at least).
I'm trying to remember some mid card WCW stuff, but I'm really struggling to remember anything that compares...maybe some other posters can refresh my memory on some good mid card feuds from 90s WCW...maybe sting/cactus jack...?
Honorable mention to Val Venus and kaentii (spelling error, I'm sure).