Even though some will complain, Heroku is still the best in my mind. Super easy to set up, super easy to scale. You can deploy to Heroku in all of 2 minutes. Best of all, it's free with basic usage.
Another thing I like about Heroku is that it has an unparalleled community using it, which means a lot of support on StackOverflow (https://stackoverflow.com/questions/tagged/heroku).
If you need to know what ratio of web/worker dynos you'd need, look here:
Heroku: web dyno vs. worker dyno? How many/what ratio do I need?
Although keep this in mind: it will be a while until you need to scale. As 37signals says, deal with scaling when you have that problem.
If you're looking for more flexibility, Amazon EC2 is always a good option. You pay only for what you use, which is always nice.
Linode is still decent, and fairly cheap too.