I think it depends on what you are looking for and whether this might be your first and only trip to the Caymans. I see you are in the Netherlands. I think LC has the best diving and that seems to be the consensus of most posters on SB. However, the diving on CB and GC are very good. If this is your one and only trip to the Caymans and you want to dive some of the best spots in the Caribbean and all you care about is the diving, I think LC is a good choice. If, however, this may not be your one and only trip to the Caymans and you like to do other things besides diving, I would recommend GC as your first trip
As far as where on GC that also depends on what you are looking for. If the best diving is your one and only criteria, I would recommend Oceanfrontiers out on the east end. I think they are a terrific op, nice isolated location feel (not quite like LC) but much quieter than the west side 7mb, and they have access to some of the best diving on GC. On the otherhand if your interest in the trip is only part diving, looking to dive maybe only once a day but would like easier access to restaurants, shopping, strolling long white sand beaches, somewhere on 7mb might be a better bet.
I have never stayed at Cobalt Coast but have been to the property a couple times when I dove with DiveTech, their dive op. I think DT was great, but I was not impressed with the location of Cobalt and would much prefer OF. We were also bused around when we dove with DT which was my only negative on DT. I think next time I dive with DT I would try to avoid the busing by simply trying to arrange to meet them at their boat. This would not be an issue if the boats leave from Cobalt but when I was there the boats were moved to West Bay. The pluses of Cobalt and DT is there shore diving options at Cobalt and Lighthouse Point. And again I thought DT was a great dive op and I have never stayed at Cobalt so can't comment on anything other than my impression having driven to the facility and having walked around the property.