In the November issue of the Toy & Hobby Retailer magazine (I just found it), they published results of an online poll which asked for the best plaything in the last 75 years (it is the 75th anniversary of T&HR magazine that month) in Australia.
Based on some of the results, I'd say that most of the voters were retailers, because some are just recent revivals or are Brands that consumers wouldn't even know the name of.
The first-release dates of some of these iconic Brands are interesting to know though.
The top 20 were as follows...
1 - Lego (1932)
2 - Teenage Mutant Ninja Turtles (1984)
3 - Barbie (1959)
4 - Monopoly (1934)
5 - Brio (Swedish wooden toy company) (1908)
6 - Sylvanian Families (1985)
7 - Transformers (1984)
8 - Schleich Animal figurines (1980) (Schleich actually started in 1950s)
9 - Cabbage Patch Kids (1983)
10 - Etch-a-Sketch (1960)
11 - Playmobil (1975)
12 - Little Tikes Cosy Coupe (1979)
13 - Lundby (doll houses) (1947)
14 - Furby (1998)
15 - Play-Doh (1956)
16 - Star Wars Figurines (1977)
17 - Slinky (1945)
18 - Scrabble (1948)
19 - Bratz (2001)
20 - Mr Potato Head (1952)
If some of those don't ring any bells, do a google-image search, and you might be surprised at what they are. (which is why I said that this was most likely votes by retailers, rather than consumers)
(beats me how TMNT got into second place)