Flights supplying Ukraine were routinely top viewed flights on that website (they were flying to Rzeszów in Poland, so there was no real risk of Russian shooting them down).
AWACS planes and tanker flying in holding patterns over Poland, Romania and Baltic Sea used to be top observed planes on flightradar24 but I should be now working not looking through flightradar24 planes over Poland ( so I will link https://www.bbc.com/news/av/world-europe-60612255 that has video of inside one of them ).
Obviously planes flying combat missions are not publishing data there. Presumably ones training in restricted airspace are not either for also obvious reasons.
ADSB Exchange even has a ‘military’ filter to focus on them.