In no particular order I would say;
England - Can only be Geoff Hurst's "They think it's all over" hat trick goal in the 1966 World Cup but his 'Russian Linesman' second goal runs it close.
Wales - Look no further than Robson-Kanu's Cruyff turn goal against Belgium
Scotland - Archie Gemmill splitting the Dutch defence in half with a mesmerising dribble in the 1978 World Cup
Northern Ireland - Must be Gerry Armstrong's goal in the 1982 World Cup against the hosts Spain, in which they ended up winning the group.
I can't think of many others that spring to mind. Some better technically of course but more iconic ? Unlikely.