If you are distributing content worldwide then you would probably rank a CDN host with edge servers higher - as it will allow you to mirror your content over multiple geographic locations for quicker delivery to local clients. Generally the cheaper operations only have limited coverage (western USA only, Tanzania only, etc), limited bandwidth available, limited storage, or hidden costs which will crop up later for redundancy, additional https access fees, lower quality analytics, or limited support. Look over any contract carefully and see if there is a downtime clause for server upgrades or DOS attacks or other backend issues. A lot of customers will pay more to get 24/7 reliable support.
Even with all of that said, simply paying the most or running with the 'big boys' as you say, will not always guarantee a flawless experience for you. Most CDNs will not make changes if you are not fully utilizing what they offer in the contract you signed. If the customer doesn't complain they will continue to charge what they have been. However, Google does alert you when the compute engine virtual machine is being used at < 5% capacity and it's easy to downgrade the server to save some money. That's the only instance I've seen where they are being nice AFTER the negotiation is done and contracts have been signed.
In addition, if you require some special accommodation, like security tokens/DRM, ability to distribute individual files >4 GB in size, don't have experience with Linux servers, windows servers, or vice-versa, specific ftp requirements or custom coding work, expect any of those to take 2-3 months to work out. If it can't be done in a reasonable timeframe, ditch them and find someone else. Some smaller "CDN" services are run by a handful of people at the controls and you might find you need to do their work for them to figure out a CORS configuration issue or send them pages of logs with documented fixes to get their service to work. Even the big boys sometimes have trouble with incompatibility with what they've always used -- as long as all their customers do the same things they have been doing, with the same tools, nobody notices that some API code for Java doesn't work with the latest release or some crap like that.
