I stayed at the LMO twice last summer. Great place - wonderful rooms, excellent (and really genuinely caring) service, great intimate feeling, and an amazing spa if that's your thing (it's not mine, though). Overall, I would stay there again anytime I'm in HKG.
We were considering staying in some of the others (for a time we had reservations at the FS, MO, Pen, and LMO for the second trip), so we took a look at them all:
MO - very nice, but big and not as intimate as the LMO. The rooms are more traditional MO and less "zen-style." Mostly, though, it's just bigger and we liked the much more intimate LMO. Plus, they don't have the airport meet-and-greet of the LMO which was a great perk of the former.
Pen - nice, of course, but we didn't want to be in Kowloon, which we realized fully only after we visited it. Some people do really like Kowloon though. The rooms at the Pen were nice but traditional.
FS - I didn't see rooms here, but the public areas were really nice. The problem with it for us, though, was that it is very slightly out of the way of the main part of Central. It's not a big deal as HK is very small, but we still prefered being more in the center at the LMO.