My Top 3 in heavy build quality are as follow:
Telefunken Hifi Studio 1 - man what a heavy unit and full of quality!
Panasonic RX-A2 - not a real boombox, but certainly worth a try. i opened one of these at a time and never got it back together. my plan was to reach the tapedeck - never try this at home guys. what a mess - i think im falling back into this nightmare again. now its my spare-parts-box
Sharp GF-999/1000/777 - one of these or similar are very well build, high quality but at least rather easy to handle when broken - not like the 2 boxes mentioned above
I also agree on Aiwa (especially CS-880) , RX-7700 and the JVC PC-series.
as far as it goes - you'll nearly never go wrong with a silver box from well known companies - the black boxes look like low quality and what a surprise - are.