A site being popular is a double edged sword. You have more opportunities but it also means you are competing with more people. It is much harder to make yourself stand out.
Go ahead and use Indeed and Monster and Workopolis. If you are unemployed, you have all the time in the world to register for ALL the sites out there. In addition, you also want to look for companies you want to work for. Many companies have their own job posting section on their websites.
If you are already employed and looking to change job, I'd make more use of recruiting agencies. They can save you time.
I've mostly used Indeed & LinkedIn. LinkedIn is the platform most recruiters have contacted me through.
I've also had some interest with Hired but that's more of a slow process as you submit a profile and the website will try and match you with jobs/recruiters that meet those needs. There's also a limited time they will promote your profile to recruiters so it's better used as a more passive approach to job searching.