I love Poirot! Just finished watching the entire series recently. There's some really great British murder mysteries series out there--some of my more modern favorites that I'd recommend to just about anyone are Endeavour, Vera, and Foyle's War. The mysteries in Foyle's War tend to be less complex than the other two, but it's more than made up for by Michael Kitchen being perfect in the lead role and the fact that the period setting (WWII in a coastal English town) isn't just an aesthetic choice but is integral to the plot of both individual episodes and the series as a whole. For all of them, the production values are just on another level compared to the slop churned out week after week, year after year for American police procedurals.