the main scenarios (and none are imminet atm)
in no order
Israel on Iran
India v Pakistan in re their border regions
Russian tactical use in Ukraine if Putin thinks that he both can get away with it AND that it is his only main functional option left to fully winning it back
North Korea if they go full death cult and Kim Jong Un thinks he is going to be taken out (targets would likely be SoKo, Japan, and (100 per cent suicidally) the US, I wager)
Iran (whether directly or through the use of proxies) IF they actually can get them made and they feel that they are going to be flattened (the Mullahs are NOT suicidal, so they would have to be 100 per cent convinced they are about to be taken out)
China tactical use on Taiwan if they decide to take it back and feel that (like Putin above) they can both get away with it AND that it is their last best option for a quick conclusion to the takeover
Sunni rando terror nuke(s) (likely sourced from renegade elements in Pakistan) The targets could be anywhere (US, Europe, Middle East, Afghanistan, India, Russia, etc etc, even Africa)
Samson Option (launch all 400+ thermonukes globally) by Israel if they think they are going down (least likely of all of these)
wildcard
the UK finally settles the French question of the last 1000 years (I jest!)