Gosh. There are quite a few. Here's my list:
1. Edge/Lita/Matt Hardy
The ultimate in love triangles. It had it all. A perfect female and male wrestling duo in Lita and Matt part of Team Xtreme. An unfortunate injury taking each out at separate times caused them to be separated for extended periods, and Edge was married for the 2nd time. But the fight between Kane and Edge had an uninvited person, Matt, and no one could foresee what was coming, and WWE just HAD to swoop in on it. Leading to some INTENSE and bloody battles. Who could forget the Summerslam 2006 where Matt was busted open so bad the ref pretty much stopped the match before it even started? Then the Unforgiven 2006 where Matt even took out Lita in the steel cage after he took out Edge. All the different run-ins and call ins and back and forth. I think people were so shocked to see that side of Matt, and shocked by Lita's actions since she was supposd to be the sweetheart of the Xtreme Team.
2. Orton/HHH and the McMahons
When Orton was systematically taking out each and every McMahon. I literally was getting chills down my spine when I saw Stephanie show up to the arena that one night. Then after his match with HHH, he handcuffed him and Legacy was beating him. He demanded Steph come down to the ring, and that was it. I was biting my nails nervously. To see him ddt her unconcious and kiss her in front of HHH. That would drive anyone mad. It was increasingly amped up when HHH got revenge and broke into Randy's "house" one of the following weeks.
3. Matt and Jeff Hardy leading up to WM25
I was so SHOCKED. When it was Matt who was being Jeff's accidents at the different shows. The car accident, and pyro accident, the attack in the hotel. I think no one saw this coming because we were all so waiting on Christian to possibly show up. Then Matt's creepy promos about Jeff's house burning down, and Jeff's dog dying in the fire was so rubbing salt in the wound. Especially for family, and if you've read their book, and seen how well they work together. You know this would be hard for any fan to watch.
4. CM Punk v Jeff Hardy leading to Jeff leaving WWE
This was one of the BEST ever. From the moment Jeff defeated Edge at Extreme Rules that one year..Then u heard Punk's music like seconds after the win. Everyone's heart sank to their stomachs. You could tell this was a new can of worms. From that point forward, it was survival of the fittest. I don't think I've ever seen any crowds more invested that with those two. Especially with something as personal as Jeff's past, and CM Punk's gimmick of straight edge. That really can ring out to the true fans, and can be almost seen as a slap in the face or saying 'see ya later' if someone was thinking that is a way to get rid of someone. But even after Punk banished Jeff, to continue the barbs and dressing up as Jeff Hardy, he knew how to still make things very intense and personal. i still will keep this in my most intense of all time.
5. Jericho v HBK 2008
This was a very good feud. And to start Jericho on his all time best streak of personality, although I still like the old Y2J. But, this had it all, the assertion of a new personality, and trying to prove oneself. I think the nail that put it in the coffin for me was the involvement of HBK's wife when Jericho punched her on the stage at SummerSlam.