it's not necessarily the best tool (I don't think there is one BEST tool) but I really like REST Assured (http://rest-assured.io) for writing tests for RESTful APIs in Java. It's easy to use, pretty powerful and the tests are (to a large extent) human readable even though it's code, thanks to the given().when().then() syntax.
Try SOAPUI and POSTMAN.
Ready API! Pro for functional testing RESTful APIs. Also, there are robust materials for training on the tool via the smartbear"dojo".
I think you should try Robotium Android Testing Tool I have used it for SmartIO App did helped me alot, It has lot of good option for quick fix of bugs.
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