I'll go with what'll probably be the least-chosen and say modern. I just like the artwork these days, though there is a definite "charm" to all the other ages in many ways. The stories these days are more complex and engaging to me, too, though the huge crossovers can get a little overbearing at times. I only read things like GI Joe and Transformers when I was young, so I had no attachment to characters who had been around for years and years.
I still would love to have some first appearances of some of the more popular characters (Green Goblin, Rhino, Juggernaut, Carnage,Wolverine and the like) that span several ages, but I'll have to go with modern.