He was one of the major proponents of the Bauhaus style which wether you are a fan of it or not was quite influential 30-40 years ago. UNESCO and the Whitney Museum are two notable buildings. And he came up with this In my opinion, there's nothing much great about Bauhaus in general. To me it just another soulless modernist style that I'm happy we are rid of.
Well I too am not a great fan of the Bauhaus influence, but wether you like it or not it had its merits. Breuer revolutionized chair design by being the first to introduce tubular steel into his chair designs. As mentioned earlier his two most famous works are probably the UNESCO and the Whitney museum.