There is no best marketing strategy for ecommerce because every business, every sector and every customer is different.
Some sectors don't work with Facebook or Instagram because that is not where their audience hangs out. Others may be better with Adwords, YouTube, SEO or even Chat Forums.
As Mark says, the best strategy is the one that engages the RIGHT customers and delivers based on their needs.
The important thing to remember is that selling is a process and not a 'one-hit sale is in the bag' scenario.
This is where many many businesses fail. They throw some cash at adwords, it doesn't work, so they give up.
The first step before anything is to identify your target customer and understand their needs, what makes them tick, their lifestyle, whats important to them, and what their pain points are. Think what keeps them up at night!
You then map out the customer journey so you can understand their decision-making process. I've found the best way to do this is to actually talk to them (another thing most businesses don't bother with).
Once you have an understanding of how they think and what is important to them, you can create content on your website that solves problems, gives answers or inspires your target audience. This could be how-to videos, expert guides, trends, Q&A, Expert interviews, product reviews, interviews with other customers, product tests, etc.
The purpose of that content is to engage them so they stay longer and want to hear more from you. Look at the gadget sites as a good example on how to do this.
The main goal of any marketing campaign should be to get people ONTO YOUR EMAIL LIST so you can start a conversation and build a relationship with them.
If you do this instead of pushing for the sale you will build a more loyal customer base who will spend more and return again and again because they value what you stand for and the value you give to them.
This will make price almost irrelevant and will mean you waste less in marketing spend and wasted website visits.
Yes, yes, I know it's not that easy but it's the correct way to build a brand and a sustainable business. If you focus on price and have a homepage that vomits offers, then you are dead in the water before you've even started.
Focus on being HELPFUL and VALUABLE.
If you sell products that are available elsewhere then focus on being a personal brand that shows expertise and 1st class customer service. Be transparent and be someone people can trust.
Too many websites are faceless which makes them untrustworthy and people just don't buy.
Know your customer. Be niche and don't try to be all things to all men. Go narrow and broaden your focus later.
Once you know this, have created the content that they will find useful, then your adwords or any other type of marketing will have far more impact and be more successful.
Rome wasn't built in a day and neither were the successful ecommerce brands. Get the foundations right.
You can't throw money at it and expect sales overnight.
Check out Beardbrand for great example of how to do it. Also Google 'Eric Bandholtz brand building on reddit' to see his approach.
He's the guy you should be trying to emulate.
I hope that helps.