Which keyword research tool you prefer the most?

Try wordstream keywords. Its limited to 30 searches though.  

http://www.wordstream.com/keywords

Yes sir its a good tool. Earlier I too used wordstream but now I prefer adword tool much benefecial. Have you used wordtracker?

Yes Nikhil.......I used wordtracker.......really efficient keyword tool

you can also try KeywordDiscovery and Overture Keyword Suggestion tool........

http://www.keyworddiscovery.com/search.html

http://www.vretoolbar.com/keywords

should we prefer long tail keywords according to the latest hummingbird algo?

As far as I know, it is nothing related to long tail keywords.

Read this. 

In paid,Market Samurai is best.

Do u want to use trial market samurai tool lifetime then pm me :)

You can try  semrush.com , it provides very good tool to find competative keyword and total searches i goggle. I also use it and you can rely on it. 

I use paid version but you should give a free try once.

Hope this help.