The best Photoshop was the one were you first thought 'Yeah, now I've got the hang of this'.
So for me, Photoshop 4, where layers were first introduced (I think) was pretty cool. Version 6 where they introduced the top toolbar was also one I remember fondly.
But to be honest, I find it difficult to keep up with all the new features that are introduced with each version. Too busy at work to set aside time for familiarising myself with new software which is a shame.
Also, it seems to me that it's only rarely that new features come along that are earth-shattering. Layers, masks etc to me were far more fundamentally important than the healing brush but then I don't do a lot of retouching.