Went through this one recently. Have had (and still got) a Pioneer plasma from 2011 which at the time, was supposed to be the best TV money could buy. Through work, I got to see quite a lot of TVs and was of the opinion that was a fair assessment - a great TV quality wise.
New TV would have to be good to be worth the change so I did some research. Having kept an eye open I'd already pretty much concluded that OLED would be the ideal replacement for the plasma but to my genuine surprise, I ended up with a Samsung Q-LED. I honestly didn't expect to ever buy an LCD TV, but the things they have done to mitigate the problems of LCD are impressive. The final thing that did to for me was getting the lad at Currys to put the same material on an LG OLED and a Samsung Q90 right next to one another - the manufacturers don't like you doing that! The LG OLED had fantastic black performance but the screen was way more reflective than the Samsung, which somewhat offset that in most viewing situations, but the Samsung made an absolutely incredible job of processing some iffy source material - it looked rubbish on the LG OLED but amazing on the Samsung. Yes, I know it was post-processed....but I don't care, it looked great! The LG showed terrible contouring, the Samsung replaced it with a smooth graduated field of colour. HDR material looked good on both as did UHD. OLED slightly better for the absolute image purist still, but I don't watch most of my TV in perfect viewing conditions, so the Q-LED won out for me. A big bonus, I never have to worry about the potential for screen burn. Always ran the plasma a bit on the dim side to try and mitigate that risk. Would likely do the same with an OLED just in case. No such worries with the Q-LED, so I can spend hours playing on the console without worrying.
Samsung & LG both make a good TV. Probably a good idea to get along to Currys or the like and have a play with the different 'smart' systems - see which you prefer. The Samsung one gets a bit of a blasting for the voice control, but the main system seems pretty good to me.
Also, consider looking at a model from last year - you can usually get a better TV for the same budget as this years models that way. Won't have the absolute latest features/signal processing/etc but that isn't always a bad thing.
Another factor in favour of the one I picked was the Samsung "one invisible connection" setup on their top models. There is a black brick that takes all the inputs then a thin (about 3mm) translucent cable that connects to the panel itself. This carries the signal and power for the panel so saves masses of cables behind the TV itself. This worked out great for me, as the old Pioneer had a similar external 'media box' setup, but with a Displayport and mains cable to the panel itself, so I just swapped out the boxes and wires.
Can't hurt to wait for the sales, but you don't tend to see much of the higher end stuff. I kept a close eye on prices for a couple of months using PriceSpy, then saw a good offer on John Lewis with a £125 voucher refund so jumped on it.