This is a topic that is just soaked through with people's different preferences and biases, based mostly on experience and comfort. It seems there are better and there are worse, but anyone you ask will probably recommend what they learned on, and what they learned on will probably be what they still use today.
The best analogy is asking guitar players about their choice of instrument, because honestly, beyond the level of total crap, it's as much the person as it is the hardware. Littman is basically the Fender Strat of stethoscopes. They're very good, and they're thought to be the gold standard because so many people use them. However, there might be perfectly good reasons to use something else.
I have a DRG Puretone Traditional, and love it. Heavy tubing with steel spring material inside. In truth, I can't say what is objectively better, but I like what I've got. (I also have a Carvin Custom all-mahogany with a maple top and active electronics, and think Strats feel like holding a two-by-four, so there you go).