The Night of Decree (Qadr). I will hopefully be able to do this this Ramadan. But then again, only the MOST devoted Muslims can receive the benefit of Lailatul Qadr, one of the last 10 odd nights of Ramadan.
Messenger of Allah (pbuh) said: “Verily, it is during the twenty-seventh or the twenty-ninth night. & verily, the angels who are on the earth during that night are more numerous than the number of pebbles.