If you're on Windows, Microsoft is edging (read 'diving headfirst into') towards requiring a Microsoft account just to log in. The kicker is that they are extremely bad at mixed account scenarios, you know like needing a certain account for work and another account for private stuff. Enjoy the entropy!
I never really thought about that Google is surprisingly good at this, despite being more complicated: On some services you can have multiple identities even within the same Google account (like on YouTube).
They have a "Switch Account" button and it just lists all of your Google Accounts with all of your identities (YouTube channels) below them. It's really comfortable to work with and I hardly (well, not never) mess this up. The identity icon in the top right that is also repeated in every place where you're submitting something with that identity (like next to the comment box) helps to keep you oriented.
Typically I give Google a lot of shit, but they've got this figured out at least.