There are several criteria for bandwidth quality. Technical criteria such as latency, packet loss, quality and quantity of peering, etc.; and subjective criteria such as 'over-selling' , back bone capacity, and carriers that have restrictions such as push:pull ratios.
Further complicated when looking at carriers alone, versus companes that mix multiple carriers using decision criteria such as Internap or GNAX.
Another factor is traffic destination. If you have a webserver in the US and your traffic is destined for Europe you would want a different carrier (and different datacenter) then if your traffic is destined for Asia.
Finally, look for a data center that has multiple carriers. Thus if you arent happy with one, add another.
I think that bandwidth quality is a lot more of a personal opinion, some companies swear by level 3, and others swear by Peer 1, or even Global Crossing. I personally prefer Peer 1as a primary and Global Crossing as a back-up. Definitely go over the latency and up-times of each colo. I don't think you need to factor in just the bandwidth provider, but also the colocation comapanies effectivesness to give you the best service. Seek out colocation companies with an enhanced internal network that preferably use HDLC, or a virtual gateway for an immediate flip should one connection go down.
Your biggest inquiries should be how many companies do they peer with, how is the internal network set-up, what is the average down-time, and ask for some real-time stats that they are showing from other customers. A lot of companies provide a looking glass app that will let you analyze the network performance yourself.
When it comes to pure quality, I would avoid the value companies that offer a single provider, since you are asking for downtime. You really need to start contacting companies and asking for specifics. Some are great and some are pretty horrid. Just keep in my mind that sales guys can really push some good looking numbers at you that aren't correct, so ask to speak with the engineers.
You also need to factor in what you are doing, and how important latency and speeds are vs. uptime and stability. I know companies that can give you some insane latency latency and speed, but the connections do drop.