In my experience AGENCIES are the most effective way of getting a job. Use a few of them at same time. Second would be Linked-in - that get scanned by agencies every week if you leave your "Actively applying" switched on.
Next, Linked-in is full of jobs sites and agencies still contact me because I came up in their search - i.e. the keywords in my profile triggered their search bar when trying to fill a vacancy in your region.
Job sites - the least effective. I read they were the least effective, as by the time you read it, dozens have already applied. I remember applying for as soon as my email was triggered with a notification, but found 100 other people had also applied which means they won't even acknowledge your application - which is the one thing that will cheese you off!
Surprisingly, I found the big name job sites - the ones that advertise on TV - to be the worst in terms of laziness or ability to listen and understand; they were just stuffed with a load of young grads called recrutiment specialists without a clue. Smaller, niche IT ones need the money and work their socks off for you.
Make sure any covering letter or CV shows that you have read the job spec and even though it hurts, leave out irrelevant stuff and waffle. Don't list responsibilities. Summarise a responsibility but the majority should be quantifiable achievements in percentages or positive outcomes if you can't quantify. Delighted customer, won new work worth.... Try to write each achievement in 2 lines and no more than 4. It encourages you to use few and better words to say key things without wasting space.
Also refresh your CV every month, as the mere fact you edit and save a CV online (such as technojobs.com, for example) sends a notification trigger to all the agencies that subscribe to that site or more usually the syndicate they themselves are partnered into. Yes, you can often get a lot of sales trash as a result of posting your CV.
Agencies that impressesed me (I only speak for UK) were MODIS, NINENAILS, NEWMAN STEWART, HARVEY NASH, GREENFIELD RECRUITMENT, X4TECHNOLOGY, CELERITY SEARCH only because they bothered to get back to you or understood what you wanted and didn't keep asking in the graduate voice: "What does that mean?" when you list skills that are not windows 10.
Use lots of agencies and keep going; you owe them nothing. They earn money from you, so its in their interest to work on your behalf and the more you have, the better. But - and I can't stress this enough: Be absolutely clear what you are looking for, the more the agent understands this, the easier their job is.