I don't see them very often, or maybe more likely, I just don't know the chars from the other property and thus don't even realize when they happen lol. I've seen #5 and #1. The Toriko/One Piece eps were light fun. Isekai Quartet is great, and I'd probably consider that the top since crossover is the entire idea behind it
I just wish they'd hurry up with a season 3 with more shows being mixed in.
I remember seeing a few other crossovers that were super low key. One that comes to mind is in Haganai, when we see the Oreimo characters at Comiket:
lol when I was searching for the image, I found a page of cameos. Don't remember this one from Full Metal Panic, but it's pretty amusing:
