First off, SEO is not a fast route to success. Real SEO takes time. So asking which technique will get you fast results is asking the wrong question.
Second, there's no such thing as a one-size-fits-all "best" tactic for link acquisition. Anyone who tells you without reservation that there's one technique superior to all others in all situations has essentially just revealed they know nothing about SEO. (Regardless of how much they may proclaim themselves to be a "guru" or "expert.")
In general, you want to get links from high quality sites. So, don't waste your time pursuing links from places such as free-for-all directories that nobody ever visits, or blog networks full of spammy "spun content" blogs.
In order to attract these good quality links, you have to have something worth linking to
. So the first step is to make sure you've got awesome content. Unique, interesting, informative, entertaining... not just a bunch of "spun" articles, stuff downloaded from some article directory and cut-and-paste copies of product vendor information.
Then, it's best if your inbound links come from pages that your target audience will actually visit and see. That way, they can deliver you direct traffic as well as SEO benefit -- and that direct traffic can help sustain you in the event the SEO benefit ever goes away for whatever reason.
So target sites that serve the same demographic you want to reach. This means you'll have to put some time into doing specific research related to your market. You probably won't find a handy-dandy list of "good" sites to get links from just waiting out there for you.
Finally, if the pages you get links from truly are high-quality and generate direct traffic for your site, you'll find you probably don't need as many of them as you might have initially thought. The people who say you need thousands of incoming links only believe that because the links they get are generally of such low quality, it takes a ton of them to make any difference. When you have good quality links, it usually doesn't take thousands to get you the traffic your site needs to be successful.
How to tell if your links are high quality? If they were easy and free, it's almost certain they are not
high-quality. Good quality links are costly -- either in money or in the effort it takes to acquire them (or both).
I know this sounds like a lot of hard work. That's because it is. SEO done the right way is not easy, and isn't supposed to be.