Which watch is compatible with iPhone 7 Plus

No, Watch Series 8, SE 2nd Gen, and Ultra all run on watchOS 9 and require a phone running iOS 16.

An iPhone 8 is the oldest phone that is able to update to iOS 16.

I don't know how old of a watch you'd have to go back to in order to be able to pair it to your iPhone 7 but you would have to be certain that the watch you get can't be updated to watchOS 9 because it's the watchOS 9 that requires an iPhone 8 or newer.

Hopefully I didn't confuse you, I've tried my best.