Google Keyword Tool (https://adwords.google.com/select/KeywordToolExternal). The others reply on questionable data sources. While they do may research easier, I've found them to have wonky results. Just my preference, though...
I use Google adwords keyword search tool. this allows use to modify your search and display various columns. You can also use it to search your website and it will return keywords that might apply to your site.
I think there are now good keywods tool.Keywords depend of your content information: H1 tag, alt tags and words in body. You can use best keywords but if them wouldn't be related to you body, it dosent matter.
I like to use a variety of keyword tools from Google Keywords tool, wordtracker, seobook and keyword elite all at once so I can work out an average and go with that.
I cross reference between Google Adwords and Word Tracker. Using two gives you a better idea of what real traffic will be.
i think that 90% of webmasters use Google service , so you should try Adwords Keyword Tool External .