Rajpipla State is ideal if you demand completion. The State Post Office opened in 1880 and closed in 1886, after issuing just three stamps:

Not cheap, though. The 2014 Gibbons prices the set at £55 mint, and £275 used. And forget about finding these on cover. The 1 Pice blue turns up very occasionally on postcards - and sells for around £500.
And of course, few things about the Indian States are ever simple. Gibbons lists an imperf between of the green 2 Anna, at £950.
Still, you can ignore the imperf between pairs, but you might want to add a Rajpipla lettersheet to your collection:

These turn up for sale fairly regularly, but some sellers have rather inflated ideas of their value.
And lastly, if you're tempted by cut squares,
don't. I was taken by this cut square of the 3 Pice lettersheet

because it was used. Should have done my homework before bidding on it. There was, apparently, a small cottage industry faking these things, even down to some artistic scribble on the back:
