Your question is too generic and not specific enough. There are plenty of relevant websites accepting banner advertisement depends on your niche, thus some banner advertising broker with a comprehensive search system to help you find the best advertiser in your specific niche / keyword. You may consider to check this one - Banner Advertising on a Fast Growing Network!
Maybe try to buy some banners from exchange banners programs, they should have low ratio for advertisers
There are several places that you can advertise with banner ads that are a lot cheaper than Facebook. Traffic Exchanges like EasyHits4U (very responsive) allows you to place typical 468x60 banners into their exchange. You'll need to purchase credits in order to get the banners seen, but they're extremely cheap: $20 for 5000 impressions or something like that. I once put an iTunes banner in there and made a couple of sales. These ads usually require a 1 day approval, but once they're approved, they're ready to roar. Similar to Traffic Exchanges are Text Ad exchanges like Ad Exchange Pro. You can join for free and never have to pay money to advertise unless you wanted to. But... I recommend getting a separate gmail account when you sign up with text ad exchanges because each day you'll receive any where from 9 - 20 solo ads in your email box. Don't delete these. They have a link where you earn credits for viewing. When you join a text ad or traffic exchange, you might be surprised to find other traffic and text ad exchanges being promoted. So people tend to join several of these. In each one they will typically advertise their favorite product, and then they would promote their ad exchange referral link in another ad exchange. The more sites you've joined, the more you can advertise these referral links so that you keep building credits. I wouldn't go overboard because you can lose track of how many you've joined, but it's great for people who have virtually no ad budget. Some ad exchanges are designed differently than others, so be sure you understand how they work. Lately, there's been a comeback of ad exchanges that run traditional advertising where you simply buy impressions, but they are actually placed or are targeting websites within your niche, sort of like contextual advertising but way, way cheaper. These are the best ones and are usually worth upgrading for better privileges. You can also earn extra money from the advertising. So these Text Ad and Traffic Exchanges are very simple all-encompassing advertising method/affiliate systems that used to have a bad reputation for their low quality traffic, but they're starting to get better. Try em out.
That's very hard to tell the "best" without more specify information If you are in low pay niche, adwords is good and the price is not expensive However, if you are in high pay niche, you might consider bidvertiser
Banner efficiency is dropping every year, cause net users are getting used to skip any media ads from their attention and concentrate only on the content. So if you need visitors, better go for contextual text ads or viral marketing. The best is organic search results, of course, but its hard to get there and, even if you are on the first position, it may be just not enough traffic, right?