What is the best Automation testing tool available for testing automation?

I think that it depends upon application need and also what type of automation tool you are choosing to test the application and how much are you efficient to test the application using that automation tool.
In my opinion ,currently selenium is a very good free source automation testing tool to test the functionality and there are lots of things available to learn from here.
If you want to do performance testing then you can choose Jmeter and this is one of the best performance measurement tool.

Every tool is not best for every application or each testing so it should be very specific that which technology is using and for which testing then we can apply tools according to that.

it depends on the type of Prject and project fund you are having.QTp,selenium are good tools...Each of them have its on advantages and disadvantages.However I prefer u can use Selenium as it is an open sourse tool developed by thoughtworks

The best tool is a real human being with a good knowledge in DCS, PLC, HVAC&R and electrical and controls theory.

Otherwise there is no single piece of equipment that can do what a good control engineer can.

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Selenium is one of the best open source tools

The application under test, budget, skill set, time availability etc have very important role in the selection of automation tool .
For web or windows application QTP is a good choice.
QTP is one among the robust and leading automation tools available in the market .QTP support most of the applications .
Since it is from HP , the support is also good.
Selenium is a good option for web applications if you are considering an open source tool Some of my suggestions are , Web and Windows Applications : QTP Webservices : soapUI (open source) / soapUI Pro(Licensced) Performance Testing : JMeter/ loadUI Mobile apps : SeeTest /Eggplant/ Robotium/M-eux

Did you try iMacros?