I could go on and on and on!
There are so many. I am reading Agatha Christie right now. She produced so much that it is nice to not be dreading running out. I think her best is And Then There Were None
which is an all night page turner (but no Poirot).
My favourite mystery of all time is Caleb Carr's The Alienist
If you like spyish mysteries, I stand by Tinker Tailor Soldier Spy
as the top of the list. It is magnificent (as is the 1979 BBC TV adaptation, and the recent movie is great too).