I have used Mouser, but have found that Digi-Key has cheaper shipping, and they're faster too.

Sometimes only Mouser has the component I need.

 

Having it in stock is what it is all about. Digikey apparently has a HUGE inventory but not everything. When I was using Arrow last year, several times they did not have what I was looking for. Long ago, Jameco was my goto as they had a lot of semiconductor "stuff/chips" that others didn't. Some of the smaller distributors, like Mouser and Spark, carry some "specialty" items that the big distributors don't.

 

I also find that Mouset still carries components which have already been obsoleted by DigiKey.

But I do find DK’s component selection tools and filtering capabilities the industry’s gold standard.

The worst component selection tool? Amazon. By far.