1) SEMrush - I believe that among all the 3rd party software, SEMrush has the largest keyword database. Their search volume data is pretty accurate and aligns with the Google keyword planner. Also, based on the type of content that needs to be produced (i.e. informational, transactional, etc.), one can utilize different filtering options available in it.
Before you can put it to use and start making Long Tail Pro pay for itself, you first need to understand exactly what Long Tail Pro does and how it will benefit you. The number one benefit and feature of Long Tail Pro is that it helps you analyze the competition in Google much faster and more effectively than if you tried to do it only using free tools.
If you're a regular blogger, these are probably the topics you blog about most frequently. Or perhaps they're the topics that come up the most in sales conversations. Put yourself in the shoes of your buyer personas -- what types of topics would your target audience search that you'd want your business to get found for? If you were a company like HubSpot, for example -- selling marketing software (which happens to have some awesome SEO tools ... but I digress ;-) -- you might have general topic buckets like "inbound marketing," "blogging," "email marketing," "lead generation," "SEO," "social media," "marketing analytics," and "marketing automation."
The limit on manual keywords could be higher. I personally wish we could input 10,000 keywords at a time, instead of 200. However, I understand the costs that Long Tail Pro has to maintain each time a new manual keyword is input. Not a deal breaker, I just wish the limit was higher. (To be fair, I don't think most keyword tools have a bulk manual option at all).