We prefer and suggest Long Tail Pro to all our clients. Long Tail Pro is the best long tail keyword research software online. I use their software for researching keywords for everything from social media marketing bios, to page and blog post titles, to headings (H2, H3, etc.), to meta descriptions, to YouTube descriptions, to content/articles, and so much more!
SEMrush is a very useful tool for both researching competitors when starting a site or for growing an established site. I really like to find weaker niche sites that still seem to be ranking for lots of keywords; SEMrush helps me see what they are ranking for and what I can potentially target. You can also see what keywords you’re on the cusp of ranking for with your established site - another very useful feature.
This is slightly more complicated than it used to be, because you have to actually create a live AdWords Campaign as part of the process! This puts many people off, as it looks as if you have to actually advertise in order to use the tool. You don’t — you can just setup the campaign and then pause it before it runs so you don’t have to spend a penny.
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).