I've used alot of these services. My opinions are as follows:
512MB of RAM is barely enough to do anything useful other than static hosting.
For pure static hosting, use Github pages and RawGit (which has MaxCDN support). This is free. This has been fantastic... better than S3 in my opinion.
Amazon EC2 is overpriced. Same with Rackspace. Comparable performance between them.
DigitalOcean is OK, but Linode provides better performance per dollar. Vultr is a little cheaper, but you'll pay for it in maintenance/support.
I feel Softlayer has the best physical performance for the cost. My startup is in the IBM Global Entrepreneurship program, and we get a $1K/mo credit to split between SoftLayer and BlueMix (1yr). These machines are solid.
Azure is somewhere in between. It's alright, but overpriced. We're also in their BizSpark program, which gives you a $150/mo credit for 3yrs. The same kind of "hardware" can be acquired from DigitalOcean/Linode for about 40% less. But it could be enough for your needs. The Azure portal is kind of confusing though.
I use OpenShift for some lightweight stuff, but it is memory constrained and I've experienced alot of downtime over the last two years. I used to use Heroku, which is definitely more expensive for the same computing capacity.
I also used to run a small data center before the days of everything being on the cloud. When it boils down to it, you pay for the hardware and bandwidth. It's a commodity these days, but it's still a cost. The closer to the bare metal you get, the less you share the infrastructure... meaning higher costs.
Hope that helps.