Stone Cold Steve Austin/Mr. McMahon---obvious reasons.
Stone Cold Steve Austin/The Rock---probbaly most entertaining duo I've seen.
John Cena/Edge---I was entertained by this because the two seemed to have chemistry in the ring and in promos.
John Cena/Chris Jericho---same reason as Cena/Edge.
John Cena/Batista---Perfect back story. The two reached the top at the exact same time and spent years not facing each other.
Jeff Hardy/CM Punk---The perfect time to start the "straight edge" persona and no offense to Jeff but he was the perfect person for CM to start it on. It just really flowed and hit the crowd big time psychologically.
Edge/Matt Hardy---the Lita deal was a good blend of reality and storyline.
DX/Nation of Domination---the famous "Crock" parody.
Triple H/Randy Orton(wrestlemania 25 feud)---Awesome set up when Randy attacked Vince then won the royal rumble match. Then he attacked Shane and Steph and I kept wondering when HHH would finally confront him. When HHH finally showed up on RAW coming after Orton, I literally was surprised and felt a little thrill. Then all the segments leading up to the wrestlemania match were great.
What feuds have entertained you the most?