Honestly i feel like it's mostly the foreign sections of the fanbases that still have a heated rivalry from what i can see, in countries where there are naturally a lot of United and Arsenal fans as they grew up with that rivalry. It's carnage on twitter usually between them.That's probably it.
Spurs and Chelsea are who I get excited and upset over. United and Arsenal been for almost a decade together, and Arsenal since 2009.
The only team i ever really still hate when there's no competitive rivalry is Liverpool. Maybe Leeds. In fact Leeds are a good example of what i said above, a lot of the foreign United fans don't really seem to care about Leeds.
Christ i hate their fans.
With City i've mostly conditioned myself not to care as i see their entire history post-takeover as hollow.