Each of us has different goals and interests but I really enjoy using the 12-40 underwater. I can get very close with it and at 40mm use it as a sort of semi-macro (or at least close focus) lens for small but not really tiny items and I use it for fish portraits and moderate wide angle shots like reef structures, fish schools and so on. I also like that it is quite fast to focus. I used to sometimes take a 14-45 underwater, but it was never quite wide enough. That extra 2mm on the wide end of the 12-40 makes a big difference to me. My current Zen dome will accept both the 7-14 Panasonic and 12-40 and I also take the 8mm FE Panasonic and a small dome on most trips. Those three lenses cover everything I might need on a typical dive trip, but I am not much of a macro shooter. I also have the 45 and 60 macro lenses and when I anticipate doing more macro on a dive trip, I can substitute one of those for one of the other lenses, or if I am not facing huge space restrictions, I can add the lens and port in my carry-on bags.
I will probably switch to the new 8mm and 7-14 lenses and switch out domes when the lenses are actually available to me and the dust settles on the ports and gears. One of the real upsides to the micro four thirds equipment is that I can fit my housing, focus light, strobes, all the ports, gears, lenses and camera in my carry on bags, and that makes travel more relaxed.
There are so many more lens and port choices now compared to when I started using a GF-1 underwater...it's like being a kid in a candy store.