Lego - it has already been voted the top toy of all time and its popularity has only increased. It is the versatility that makes it so great, being only as limited as the imagination of the child. The world of Playmobil is also timeless - I had my own collection a a child and today my own kids love it just as much. It's good quality as well, and so can last more than one generation.
Also, I think every child should have a bike or a scooter - it gets them active and they are indeed timeless. The simple toys that have been around for generations are hard to beat.
There are many other timeless toys or course, but I have just mentioned the first that spring to mind and which are enjoyed by my own children just as much as they were by me.
Toys that are not timeless are those which have limited play value and which are related to passing fads, especially current movie merchandise and characters from tv programmes which will not have staying power for the future.