Probably my answer would vary depending on whether you asked me before I had done liveaboard trips to Galapagos or Maldives or Burma or wherever, but currently I'm curious about the possibility of a liveaboard to the Northern Hawaiian Islands. I'm not even sure if there is such as trip available, but it's the only way to visit, AFAIK, whereas at other places I want to go on a liveaboard, such as Raja Ampat, it's at least possible to do it land-based.
I like liveabaord diving, so my LOB list can't really be reduced to just one, LOL. Realistically, my most likely next non-work-related LOB trips will probably be Komodo and Sipadan, but more out of convenience than the fact that they're at the top of a wish list.