Feels like I've done tons of quiet parks, to the point where some trips I feel horrified when I get somewhere and actually have to queue because I'm just not used to it at the time. A few particularly standout moments/places come to mind:
Wiener Prater - I got here earlier in the day than most people seem to and I was pretty alone other than the various ride staff, most of whom were quite rude about having to take money off me and actually run the rides. It was rather disgusting and not a real park.
Wanda Nanchang - There were more people outside the entrance plaza trying to sell knock-off tickets than there were guests in the park. Honestly can't recall seeing more than 1 other guest couple on this day so most of the time was just us and the staff - who were all very excitable (uncharacteristically so for China) and happy to see someone walk by. Took several solo rides on the 2 major coasters and a solo ride on the drop tower - a particular highlight as the ride operator was so happy to have a customer that she took me by the hand and skipped along the queue with me. Lowlight was that they refused to run the caterpillar cred without a full train for 'safety reasons', and filling a train was impossible, so spite.
Oriental Heritage Jinan & Xiamen, Fantawild Dreamland Zhengzhou - Everything dead, max 50 guests on park, endless empty open spaces. Solo rides on Jungle Trailblazers with varying levels of friendliness shown by staff from Zhengzhou who couldn't do enough for me, to Xiamen who hated having to deal with me.
Hansa Park - First time I went here it was extremely empty with a lovely quiet and special atmosphere. Don't think I ever did a ride on a full train and I had the whole Karnan preshow, loading sequence and ride to myself on my first few laps which was equal parts terrifying and amazing.
Japan has a lot of parks that seem to be dying when you go, but most have seemingly survived for decades like this. Of the more major attractions:
Himeji - No more than 20 people on park? Never open more than the front row on their B&M.
Tobu Zoo - Only people in the park, appeared to open the Megalite specifically for us.
Universal Singapore - potentially the only one in the chain that you can ever get a truly quiet day on. There will always be people milling around in tourist fashion, but ride wise I've had it empty enough for solo rides on the Mummy, which are awesome - love sitting alone on a spectacular dark ride thinking how much money is each ride cycle costing them? Just to please one guy.
Six Flags New England can't have had more than 50 people in it on a rainy monday. 3 laps on the RMC without leaving the seat. They did however handle things in disgusting fashion after that.
Thorpe Park used to have quiet days but it seems impossible now. I remember a time when it was so quiet we were challenged by a ride host to stay sitting on any ride in the park indefinitely, every time any other member of staff asked us to.
Can't have nice things any more. Go on a September weekday morning and wait 30 mins for all the coasters cos they're stingy bastards.