Some will tell you seo and organic traffic are the best, but truly it depends on what niche market you are in.
If you are involved in a market that is saturated with competition then an organic SEO campaign may be a futile effort especially if you are first starting out and have limited funding.
PPC campaigns are another beast altogether. While it is a great way to drive traffic and close deals it is a trial effort approach. To run a successful PPC campaign you will need a good amount of funds so that you can split test advertisements and landing pages to see which ones will work best for you.
The best kind of traffic in my opinion for a start up business would be traffic you receive from backlinks. While not as targeted as SEO traffic you can rake in a pretty heavy amount of visitors in a short period of time using this method. There are countless ways to generate backlink traffic, but the easy and most cost efficient would be posting articles on your subject with you website in the sig, forum posting, and answers websites.
These are good places to start generating some traffic and maybe even a few sales with very limited funds. After you are able generating a decent amount of traffic then you may want to focus on a multi-tier strategy to really boost your traffic levels.