We've been to all 3, and I probably wouldn't skip one if we happen to return in the future. However if I had to pick one to skip, it would be Jamaica.
When we were there, we did Dunn River Falls plus a (very mild) rafting trip and lunch. We loved it so much except the distance to Dunn River Falls and getting stuck in traffic going there, and then as everyone has commented the pushiness of the local vendors. We don't have any plans to return, but if we did I might consider a raft trip that stays closer to the port. We really enjoyed that portion too, and it felt more laid back than the long trip to Dunn River Falls and all the vendors there. We ended up coming out of the river into a little bay, where we had some lunch, hung out with our guide a little longer, and the kids played in the ocean. Just the general level of crime and travel advisory in Jamaica is probably what could most make me decide to skip there.
We've been to Cozumel 2x and I don't think I would ever skip getting off there. We did Nachi Cocom once, and the other time we did a conservation type thing where we learned about coral reefs, planted mangrove seeds, and got to swim. I would love to see the honeybee farm next time we're there, and maybe combine with some ruins or a beach stop. We also go to the Mega right near the port and restock the beer we're allowed to bring onboard! And I don't think I'd want to miss GC either. Tendering is a pain, but it seems like there's a lot of variety to choose from there. We did Stingray City, and I would love to do that again, but some of the caves look really neat too.