one single piece of modern ..what would it be?  

we.. if we're going there.
I'll take the Saarinen/Girard designed Miller house with all it contains. Does anyone know this house? It's interior design is so over the top. Love it. Many other modernists I meet think it to be too gaudy.

Okidoki - I guess if all...
Okidoki - I guess if all those houses are already taken, I guess I would have to settle for Villa Savoie by Le Corbousier (on a smaller scale, I really would not mind his "Cabanon", the 50 sq m cabin he built for himself and his wife on the French Riviera....)

SDR -
I fear you may be dining alone, lol. The sideboard seems too geometric for me.
The LCW with perhaps a low rod table in white (or one of the fun patterns modernica is doing)
These two, together on a shaded patio with a great view, a good book, and an iced tea. 😀

Reitveld's sideboard is...
Reitveld's sideboard is amazing!

Yum
There are so many flavors of modern, one wants to have a number of succesive "meals" to enjoy them all.
My fantasy was always to have copies of several iconic houses, each with its appropriate contents. Let's see: Gamble house, Martin house, Pauson house, Glass house. . .

It's utterly irrational and impractical, but...
I've always wanted a Raymond Loewy streamlined steam engine. Never had a backyard big enough for one, but wouldn't it look magnificient properly lit at night next to a reflecting pool?

simple
simple answer; a barcelona chair or rather an eames lachaise to sit on, a juicy salif to get my fresh orange juice in the morning, my levis 501 to wear and my girlfriend (love is far more important then any piece of design)

Self-contained
An AIrstream Bambi.