Ok agree about the atmosphere , but I would think Liverpool or a big European match would be worth experiencing.
I went to a Liverpool match in about 1989 or 1990. Just walked in on the day (PoTG), none of this ticket nonsense - with my weekend rucksack on my back. Not a United fan by a long chalk, but was in the Stretford End. Liverpool went 2-0 up, and I refrained from cheering. United got one back, and the atmosphere was something else - even I spent the last 20 minutes or so willing them on to get an equaliser, and I can't stand them.