Basically AppCodes is now directly connected to Apple's API so our keyword popularity is pretty accurate comparing to most other tools. Sensor Tower has definitely the worst popularity estimate but on the other side they have pretty good keyword spying module so this tool can be super useful in combination with our tool.
Frankly speaking I never heard of Applause tool. As for the AppAnnie they seems incomplete in terms of keywords so anyone who does ASO professionally will probably recommend to stay away from this tool. At least this is what I would do.
TBH only you know what keywords or more importantly what phrases are relevant to your app.
Keywords by themselves can hard to rank with. Make a list of phrases in a spreadsheet then use a thesaurus on those words. You'd be surprised to see which your competitors are using.
So find the core phrases for your app and add those extra words from the thesaurus. Remember to add all the words from your phrase.
Then on your device try each phrase (the iTunes Mac app is good too but not an exact representation). You can then add your own stats to each one, relevance aka whether similar apps are using them, difficulty aka whether your best competitors are in all the high ranks. I would give your stats numbers.
Then have another stat, your evaluation, yes or no to use the phrase based on difficultly / relevance. Then sort so the best keywords are first.
Do some magic in a text editor to exclude duplicates. Then add your best phrase to your title. And the rest to your keywords.
Also bare in mind that different App Store use more than one set of keywords e.g. UK use UK and Australian. So you can get more than 100 characters.
This is my latest process, I haven't released yet. Previously I didn't use phrases. Hope this helps. I'd be happy to hear what Ibrahim Eminovic thinks.