There're many RTB platforms (hundreds of them) specialized in different ad format and area. For example, some are good at mobile app ad, some are good at video ad, some are good at targeting user behavior. One common rule to perform well in your small-budget ad campaign is to maintain your own site-list (list of website relevant to your product). For example, if I wanna sell a video game, I'll target websites like gamestop and ign exclusively instead of wasting my money on google search ad or facebook. This might not work for big budget because you'll have trouble spending all the money.