I'd get the whole run of Clash of the Champions. If not the whole run, then Clash 1 for sure as it has a historic Sting/Flair match and other good matches.
And I don't know how much you like Lucha but if you want to get into the style, When Worlds Collide is a good place to start. You have Eddie Guerrero, Chris Benoit, 2 Cold Scropio, Tito Santana and others.