I won't list all the ones I've read here as you can easily access them via this blog's index (by author) or by clicking the Simenon tag at the bottom of this post. I will say that my favourite to date is Red Lights.
Most of those I've read have been New York Review Books classics. This publisher has been steadily releasing Simenon titles since 2003. Last month saw the publication of Act of Passion. I don't have a copy yet, but here's how it opens, and here's a bit from Roger Ebert's introduction (I've heard that Ebert claims to have read a hundred-something Simenon books!).
the Neversink Library from Melville House — just a few weeks ago. I think it's currently my second favourite — a punch in the gut you never see coming, and subtler than most. Today sees the publication of Melville House's second Simenon title.
I have The President on deck, and I'll tell you all about it soon. It starts like this:
For more than an hour he had been sitting motionless in the old Louis-Philippe armchair, with its almost upright back and shabby black leather upholstery, that he had lugged around with him from one Ministry to another for forty years, till it had become a legend.
They always thought he was asleep when he sat like that with eyelids lowered, raising just one of them from time to time, to reveal a slit of gleaming eyeball.
Thanks, small and independent publishers! Keep them coming!
Some other Simenon-related stuff:
An exhibition — The inaugural exhibition of the recently opened Museé des lettres et manuscrits in Brussels is dedicated to the works of Simenon. (On till February 24, 2012.) (Via.)
A blog — The Man from London takes its name from a 1934 Simenon novel and espouses a great deal of admiration for his work. If you're at all interested in Simenon's output, you'll find browsing through this blog's archives a pleasure. (In doing so just now I'm reminded that Julian Barnes has an essay on Simenon in Something to Declare — a copy of which is somewhere in this house — which I may or may not have ever read long before I became a Simenon fan.)