This is such an amazing thread, with endless possibilities, it's so hard to just rest on one feud.
For beginners, I'm just going to address one
solid mid-card feud, from every Company out of the original three that I was used to. (W.W.F, W.C.W & E.C.W)
W.C.W.: Chris Benoit v. Booker T:
Not only was this feud one of the more heated and talked about feuds, it was a series of matches (literally) that reformed and remade W.C.W from the middle, up. This feud made the Television Championship worth being mentioned.. long before the days of Hacksaw Jim Duggan, trash-digging for Gold.. this feud was what captured my true, pure love of W.C.W (even though I'd been watching for years before this)
Benoit & Booker T had a classic
Best of Seven Series, that ironically went to an Eighth match due to the Seventh being thrown out. When you think of either man, this feud, and namely this series strongly comes to everyone's minds. It's hard to dispute this as being anything other than a very solid, pure mid-card classic.
W.W.F: Bret 'Hitman' Hart v. Jerry 'King' Lawler:
While it wasn't one of the more technically sound feuds, nor was it honestly that "great" because every match between them seemed to be so much of the same thing, it seemed to blur all together.. the fact is, this is without question to me.. the biggest, mid-card feud, of all-time for the W.W.F.
It's a feud that spanned over years. Not weeks, not months, not even a single year.. but YEARS. Yes, plural. This feud didn't just help both men to better themselves, it introduced several others to Professional Wrestling, and helped everyone who came in contact with the feuding individuals. Lawler and Hart was amazing to me, because of how it never got old..
E.C.W: Justin Credible v. Jerry Lynn:
When you think of mid-card feuds, E.C.W isn't truly the best place to try and start. Mainly because their mid-card division and Main Event division kept fricken blending so much that it was hard to keep track.
I only picked this feud, because it was one of the first I remember from when I started definately watching E.C.W. I would've picked Tommy Dreamer against Raven, but didn't know if but Raven was a Main Eventer during that time. Or even Rob Van Dam against Jerry Lynn, but I knew Rob Van Dam was considered a Main Eventer then.
Lynn had always been considered a mid-carder, through ranks, until 2000. Meanwhile, Credible wasn't considered a Main Eventer until he defeated Sabu in 1999 -
after his feud with Lynn, over that Summer.
Ending Note: I'm sure I'll think up more feuds that amazed me. I know for a fact, feuds I thought about putting were; (W.W.F) Chris Benoit/Chris Jericho, (W.C.W) Rey Mysterio/Billy Kidman, (E.C.W) Kid Kash/E.Z. Money.