Different websites giving different results- which one is reliable? We use SimilarWeb.
We typically need the following:
1. Global Rank
2. Country Rank
3. Estimated visit
4. Geography breakdown (Top 5)
5. Traffic Sources
I use Google Search.
You can write your own scraper (if you're very smart), or purchase one off the shelf.
Every website application will have its own algorithm tweak, so its hard to really trust just one. SimilarWeb's problem is that it pulls information from various other APIs and so the return is often wrong. I once launched that I knew had little traffic yield a result of almost 6k per month. What you should do is use a combination of apps along with the easiest one to measure - Google Incognito - by clearing your cache and using this browser you can search as a 'random' and get pretty good results. Unless you put a location Google Incognito tends to give global results based on keyword matching (as long as is not a local business) compare the results with apps like Similar Web, Hubspots, Neil Patel's, SpyFu, etc.
SimilarWeb and Alexa usually hit the spot. The key is to use the same one across all your searches as the margin of error should be pretty much the same as long as you use the same website ranking provider. SimilarWeb usually doubles the real visits number.