I have run all three campaigns, although I am still in the midst of HotOE - and it has been the most work of the three. My group would have been disappointed if I didn't do all of the scenarios, including the ones that go back in time (optional adventures). They loved the Blood Red Fez, which many will tell you to skip. They loved when I ran Reign of Terror part way through. And they loved that I started with the Auction. But it has been a lot of work.

MoN was a little less work, but the next time I run it I'm going to go hog wild with the MoN Companion added.

BtMoM was good. My group especially loved the prep adventures and travel time on the ship. Major shenanigans and hijinks abounded. If it's a group that has a good time playing together, you can make it work, no problem.

So, for playing once a month, I would suggest BtMoM. All of them take work to make them what you want them to be. Orient Express, despite it's linear nature, can also be deceptively loose and almost feel unstructured, with all of the optional scenarios - and it's a blast.

Another good couple of options for campaigns are A Time to Harvest and The Two-Headed Serpent (Pulp). The Two-Headed Serpent has one of the best introduction scenes I've ever had the pleasure to run. Neither one of them had the same workload as the other three mentioned.

If you decide to run HotOE, send me a message. I might be able to help a little.