I've had the Saga lounge at KEF to myself for a while. Even with a few people in it, it always seems quiet because it has so many nooks and crannies.

About an hour and a half in the LGW F lounge a couple of months back - evening, waiting for flight to GLA. I was there a bit longer than that but for a while there was one other pax there ...

I often have IST to myself for several hours at a time, due to the connecting time between KC and BA being rather long... 14 hours next month! I used to go landside and go into Istanbul for the day, but I've seen all the major sights and just prefer to relax now!

GRU for about one hour last April. Began to think that the other pax knew something I didn't.
PS thanks to the OP for I am sitting rather than the execrable I am sat so prevalent these days.

A couple of times..
LHR T1 BA first lounge once on new years day, BA JFK first lounge one evening, BA/Oneworld LAX first lounge was strangely empty once , and i had the JFK AA flagship lounge entirely to myself for 2 hrs one morning..

Annoyingly none of these have ever been when ive actually needed it quiet in order to get a few hrs sleep....

United SYD
Flying in from Hobart, too short of a transfer time to go to CBD.
First check in, first in lounge for about an hour.

I am wondering how CCR on Dec 31 around 12:30pm and 2pm will be. I shall find out. :D

Made use of the IST lounge under open doors during one of the strikes - I'd booked myself onto TK in any case to avoid any issues; the early evening BA flight was cancelled but I was still welcomed into the lounge with a colleague (also GCH, heading in another direction also on TK). Unsurprisingly the lounge was otherwise empty. Staff even went to the trouble to put out some fresh food just for us ^

Had the LHR EK lounge allllll to myself in between the 2 flights that departed before mine as I had to spent 8 hours there, but I wasn't complaining :).