I'm working on running a Memory/CPU intensive project on a cloud service, from my Googling and research it looks like I should use Amazon EC2 as there are guides it using MPI - however, reading up on stackoverflow about people's comparison of EC2 with rackspace, joyent, etc, I was wondering if this is really the best cloud option I should go with or is there an alternative better route I should take? Any insight would be appreciated.
Thanks,
Your requirements are too vague for a specific response.  It is unlikely you are going to be able to elaborate them sufficiently for anybody to provide an authoritative answer.
Fortunately for you, many Infrastructure as a Service platforms like AWS and Rackspace let you test things out extremely inexpensively (literal pocket change), so give them a try and see what works for your application.
Python Parallel Processing Amazon EC2 or Alternatives?