For two letter words:
Xu, a monetary unit of Viet Nam scores 1.90635...
For three letter words, I came with:
Zuz, an ancient Hebrew silver coin scores 1.95732...
Initially I had thought of
Zax, a tool for cutting roof slates, however it only scores 1.41824...
For a four letter word:
Zizz, to make a buzzing sound. Score: 1.84367