Targeted traffic is the best, but it depends on the niche.... if you are just looking for ad revenue then traffic in any form is great!
Honestly speaking, there are lots of websites that attract traffic if they are related to porn. It's a fact of life. Also, you could attract quickly if you use money-generating websites. With all the financial crisis going on, everyone wants quick cash.
Actually this is depend on how your promotion technique. But for me the easiest is freebies site such as coupon site. Many people will come to get the freebies thus your traffic will spike.
For me, trading sites attract traffic quickly. Specifically forex or what we call currency trading. Because nowadays, the business in currency trading has attracted a lot of people online.This business moves huge amounts of investment and can be quite a profitable.
It seems that you're not sure about your website even the content? Does it mean that you only want to build a website,but you haven't started it? I think the traffic can be got by your hard work such as SEO or promotion or other things. Maybe first,you should think more about how to set up your website.