The best network for ROI can vary from niche to niche, and from offer to offer, so you must test to find out. Since ROI depends a great deal upon how you manage the campaigns, a lot depends on your management skills. Because AdWords has far greater reach you are most often going to be able to achieve the highest ROI on AdWords, but this may not be true forever niche, so test to verify.
As far as to reach, no other network comes close to the reach of AdWords, it is many times greater than Bing. AdWords is the undisputed leader in reach. The second-largest search network as far as reach is Youtube, which has greater reach than Bing, Facebook, and Yahoo combined.