First, I am not going to type "IMO", I will write "in my opinion".
I not only use my brain but also I use some other tool,
1. Expireddomains.net,
I use it to find some potential domain name regardless of there extension.
2. Namebio.com
I use this site to check recently/past sold domain name. It's really help me to get know what are selling most.
3. Magic Keyword (A semrush tool)
I have been using this tool from long time to get some new high searched keywords in any niche.
4. Naminum.com
I love this tool, I have been using this tool after learning about brandable domain name, it's helped me to come up with 7 bb accepted name.
5. Hosterstats.com,
Simple but very helpful tool to check domain history.
AGAIN,
I don't just use my "Brain", I use some resource to get success.
The all above tool/resource are best "in my opinion" aka imo