It is hard to define best toy ever. I say this because there are so many wonderful kinds of toys and every child like snowflakes like different things. Due to this it is near impossible to call a toy "the best ever" and have all agree.
But in the realm of building toys Lego is the best. With simple bricks you can make so many many different things and when compared to its other similar toys..... such as mega blocks.... it is better. Mega blocks barely hold together.
Now there is also knects (might have mispelled that) but those have trouble looking like what they are supposed to be but Legos have found away to replicate the look of real objects in a fun manner.
So Legos are very good in the building realm.
Also personality I love legos and they were one of my fav toys... even now when I do stumble into a toysRus or some toy store (though mostly there now for video games) I do always stop by the lego isle and look at wonderful things to build.... now sure most lego sets I think are a bit over priced compared to their price tag when I was little.... but they still are bad ass...