I use only Bixby, lol.
And Bixby has come a long way than you can believe. Its learnt my accent, developed a special module for an Indian consumer. It is seemingly able to understand what I say in sleep. Lol!
Lock the screen? Yup! Turn on/off Bluetooth, screen (off only), wifi, DND mode, timer, alarm, switch off, flashlight on off, play a specific song on Spotify (partnership) through voice control, control things through IoT, and many more things. These are the ones that you are able to speak with Bixby. But it is far more than that.
If you have a Samsung, look into the Camera App and you see plethora of services under Bixby that do live in camera language translation, object detection and identification, similar object search, reverse search etc. Its like a gold mine for Samsung users, if they know how to use these services.
I do not use Google Assistant in my smartphone.
Bixby, because, it can do the things Assistant cannot. With its hardware acceleration, Bixby is able to control the hardware of the smartphone itself, to a certain degree of course. And since it is by Samsung, I trust it.
Google assistant is also cool, but I just do not prefer it in general. Although, the services a Gmail account provides is quite awesome. Makes changing phones easy for me. But Samsung has its own system developed for it too lol! And its better than Google's for sure.
Alexa is definitely out of the question for me. I already do not like Amazon Music, but I like the Kindle very much!!!
Alexa is cool to be honest, it is just that I do not trust Amazon that much, in this case.
I used Cortana in my laptop, but they have discontinued it.
And just like Apple products, Echo devices form a network of their own within 500 m or ft, with which you are subject to being tracked with outmost precision, generating your profile with literally anything that can be used to serve you. Which is cool! But scary at the same time.