You'd get a shorter response list by asking "what series that starts with a great book doesn't go steadily downhill the longer the series gets?"

Edited to add: I'm a fan of the 50-odd volume "In Death" mystery series by JD Robb/Nora Roberts. In my opinion, that one holds steady throughout. Started as a good absorbing comfort read, still is. Never Great but still good.

But mysteries benefit from their structure in that way. Each book naturally has its own internal complete story: crime committed, crime investigated, crime solved. But there's also room for the established characters, normally the investigators and their social circle to grow and develop. The In Death books are police procedurals, which also helps. You don't risk the "How many people near Jessica Fletcher get murdered every year? Is she doing it herself?" problem.