Analyzing the current top 10 search results in Google is the most important part of keyword research. This is where you determine whether or not you can produce a webpage that can beat any or all of those results. The process is known as first page analysis – and it is by far the hardest process to understand and really figure out (especially when you first start out). Don’t worry though – Long Tail Pro has detailed and in-depth instructional videos available that show you exactly how to do this.
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.
In addition, you can dig into the paid side of search and find out what keywords your competitors are bidding on, and then leverage those keywords for your own organic benefit if you're not already doing so. Search Metrics does this as well, but I've found SEMRush to provide a greater range of keywords and they save more historical keyword data than Search Metrics.
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).