Bruce appearing in the mid-credits scene in Suicide Squad felt organic leading into JL, but The Flash also appearing in Suicide Squad to take down Boomerang was a big surprise to me. The others, like Diana in Shazam 2 or Superman in Black Adam, I had an inkling of or had a feeling they would appear. Even though we knew Batfleck was in Suicide Squad in at least a flashback, I wasn't aware where Bruce would appear, and it was nice to see a Bruce Wayne scene with Waller.
Ultimately, for the surprise factor, I give it to Flash in Suicide Squad. Him showing up randomly just made the universe feel more lived in.