Excluding Hellfire(Diablo Expansion) I've played the entire series.
I honestly don't know how you could play D2 and not have played D1, I mean, come on. The whole thing with diablo had to expand beyond the worrior shoving the soulstone into his forhead, and becoming the Lord of Terror himself. Just had to expand to the Lord of Destruction.
Though both games are great, I just wished that they would have made Mephisto be the end boss for D3, then they could have expanded again, and made what everyone that plays(played) D2 wants.
But thats just what I think.