What are some of your favorite all-time sports rivalries?

Our “great rivals,” Portsmouth and Bournemouth have both been relegated to lower divisions, so we don’t play them any more unless there’s a cup fixture.

Most rivalries are based on the nearest team. Which over here would be Manchester United/Manchester City, Liverpool/Everton and Celtic/Rangers.

The Manchester derbies can be quite nasty in terms of the fans hating each other, even more so in Glasgow where there is also a sectarian element similar to Northern Ireland.

Not Liverpool/Everton, they are quite friendly affairs with rival fans sitting together. There are families where different members support different teams.

I was in the area before an FA Cup final between both sides and I saw many houses decorated in both team’s colours.

When I was a boy it was Yankees against Dodgers. We could follow baseball only by newspaper and radio. These two teams met in world series games a lot.

Sampras/Agassi in tennis although that one went bad after Agassi's biography. Now Djokovic/Nadal.

Two of the original six. Detroit Red Wings vs. Chicago Blackhawks.

Chicago Bulls Detroit Pistons rivalry.

My uncle, who lives in Kirkwood, MO and had kids attend Kirkwood High School, would say this:
https://en.wikipedia.org/wiki/Kirkwood%E2%80%93Webster_Groves_Turkey_Day_Game

I'd add:
Ali/Frazier
Affirmed/Alydar

[edit]: Just skimmed through the video and noticed that Ali/Frazier was in there.