I'm sorry but this seems to be one of the many flaws in this benchmarks. Kudos for your efforts but the graphs are almost meaningless, all things considered.

For example the EC2 instance storage is known to be dog slow. I assume that's the main reason for it looking so bad in your graphs - the picture might change with EBS.

As others have pointed out, synthetic benchmarks are a tricky beast generally. For fairness you should have optimized each host to their max potential - because that's what a regular user would do. But still, the variance in cloud-hosting makes it difficult to obtain representative figures. Slicehost might just have looked awesome in a different week...