Its fairly hard to give you an accurate answer as it can depend on where you are and where you want to send SMS's to as some providers only support some countries or have issues sending to certain networks, so make sure you know what your requirements are before choosing a provider.
In the past I've used Clickatel, as they supported the countries and networks I needed, and their API was good enough.
The only other advice I would give is whatever service you end up using pay for it, free services are more trouble than they are worth.
Hope this helps,
Jen