I remember once flying SN Brussels from BRU to SXB with AR1 or something, and there was crew of 3 lovely young FAs... and we had 3 passengers on the whole plane! That was lovely.
And the best of course was when there was still CSA's bus-flight between PRG and ZDN.. (during their winter-promo period some years ago)... I was bussing 4 times a day for 3,5days in row... and during one busride I and my teddybear were the only passengers.. on that 2,5hr "flight". There were also some when there was just few extra passengers.