That's a super broad question. The marketing channel that works best is going to depend enormously on who your target is and what your lifetime value is for your customers.
That said - some thoughts (in addition to the content marketing you mentioned): if you can build viral hooks into your product that's almost always going to be the most cost effective and scalable. The best features are those that make the product better if more of your network is involved (think Slack), but there are other ways to do this to (think the logo and link on the bottom of Mailchimp's emails).
I have had some great success in finding thought leaders in the industry and getting them to use and promote the product (hire them as unofficial spokespeople). This works really well if there are industry "celebrities" but certainly won't work in every space.
Searching linkedin (but through google's site:linkedin.com advanced search) for people that fit your target profile and then contacting (cold call, cold email, inmail) can be incredibly cost effective (I mean stupid cheap) or exceedingly expensive ($1000s per new client) depending on how it is done. I've had both experiences.
Facebook advertising is great if your target has a very strong overlap with an pre-defined interest. Google adwords is great if your targets are searching for your product already or there is very high overlap with something else they are looking for. Both of these can be a complete waste of money if those criteria aren't met.
I know people who have had amazing success on instagram. My experience has not been that.
The only way to know what will work for you is to get out there and try it.