The generalised answer imho is that different advertising media works for different businesses so there is no hard and fast rule.
If advertising is free, you need to ask yourself why? The best free advertising you can get online is good seo to get a good serp of your site with a good call to action meta description.
Or offline a press article or product review by a national paper / magazine.
Paid for advertising (online), Google Adwords is hard to beat.
Offline, imo Yellow Pages is only good if you are aiming for an elderly customer base, newspapers are really only any good for brand building, local papers need to be on front page or be a builder. Leaflets very hit and miss. Ads in newsagents windows can be surprisingly affective, but is time consuming and area restrictive.
And if you sell b2b then of course there is always FBF to consider