Are husband and wife allowed to see each other completely nude in Islam?
What sexual positions are best?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is lawful for both spouses to look at each other's naked body as there is nothing which prohibits it in the Sharee’ah.

As for the best sexual position, then the spouses may enjoy each other in any way that they like, except anal sex or intercourse during menses or confinement period. Allah Says (what means): {Your wives are a tilth for you, so go to your tilth (have sexual relations with your wives in any manner as long as it is in the vagina and not in the anus).} [Quran 2: 223]

Allah knows best.