I would have said 2001 if Triple H didn't got injured and Austin and HHH would have longer dominated as the power trip. And if the inasion angle would have been done better.
Austin was just at the peak that year in ring wise at least.
Stone Cold vs HBK at WM 14
DX parodies The Nation and the Corporation
DX invades WCW
The Rock becomes champion for the first time
The Rock vs Triple H ladder match for the IC title at Summerslam 98
Austin vs McMahon saga at it best
Undertaker vs Mankind Hell in a Cell
X-Pac and the NAO joins DX
1999 and 2000 come very close. I felt that some parts of 1999 were much better than 98, but I will have to go with 98.
From early to mid 1998 to the end of 1999. Austin was hot as hell, Undertaker had his greatest character (Ministry), DX and HHH were hitting their stride, Vince formed the corporation, Foley was taking an exceptional amount of nasty bumps and Rocky became Corporate Rock. Not to mention all the great string of matches including the Rock/Foley series.
2000 was a fun year. You've got The Rock, Triple H, Kurt Angle, the best PPVs. But I voted 1999. That's when you could really feel the attitude. You had the Austin/McMahon feud, Ministry of Darkness, DX, many entertaining stables and storylines.