I have 25 Mbps, and it is pretty good for video streaming, though not perfect. As time goes on, more and more of the video available will have higher resolutions, so over time, 25 Mbps may not be that reliable.
15 Mbps is passable, imho, though if you start pulling down a lot of HD, you're going to be exceedingly disappointed, especially as the bitrate for the HD available increases over time.
6 Mbps is marginally okay for amateur video viewing, such as YouTube, but not really good enough for something like Hulu, Netflix Watch Instantly, or the networks' website previous episodes viewing pages.
3 Mbps is adequate for low-res standard definition animation only afaic.
Also keep in mind that beyond bitrate, you need to also consider whether the service you're ordering has a bandwidth cap. An hour of television is up to about 1 GB of data in standard definition, and up to about 6 GB of data in high definition. But some service providers are cutting customers off, or surcharging, after a certain point. I've seen some (4G) limits down in the 1/2 GB range per month!!! Typical wired service providers have a cap of 250 GB (if they have a cap at all). Even with the generous 250 GB cap, you could use that up with just 10 hours of viewing per week. With lower caps, you'll hit the cap sooner, of course.
EDITED: Do keep in mind that this all assumes (basically) just one user of your Internet connection. If you are purchasing broadband Internet service for your family, including perhaps some teens or children who would be incurring their own usage, you can see how that cap could get reached very quickly... Divide 10 hours a week by two or three, eh?
However, also keep in mind that this could affect the bitrate issue, as well. So 15 Mbps service would be shared between you and, say, your teen, watching videos on separate devices, at the same time. At 15 Mbps (due to the nature of time-slicing and such) it is possible that the adverse impact on throughput from splitting the bitrate between the two videos won't be that bad (i.e., it doesn't become 7.5 + 7.5), but surely there shall be some significant degradation.
One other point about bitrate: Some service providers provide a burst service, which gives you that bitrate you purchased (say, 15 Mbps) in bursts, not continuously. That's good for quick downloads of smaller files. For an hour of video, though, that 15 Mbps lasts for a short period (a minute maybe - not sure about that) and then you're dropped down to a much slower bitrate (perhaps half - again not sure about that).