I have used a lot including Windows (Desktop) Search and Google Desktop Search. I would say I tested all of them but Everything as I never heard about this utility before reading this article. So thanks to Fireboss

I would consider Search and Replace 98 and Duplicate File Finder as complementary utilities rather than indexing and search utilities.
Copernic Destop Search was quite good but I gave up using it a few years ago as the indexing engine has remained buggy over years and next coming versions.
Google Search is pretty good but it is (was) not a standalone program with its own GUI but it uses your Web browser (IE or firefox, discontinued before Chrome was launched). Another annoying point: it would be tricky not to store your Web surf and Search History on your local drive instead of Google servers (by default!). That made me worried about their privacy rules (as usual).
Windows Search is as good as Google Desktop Search in my opinion, each of them having some pros and cons against the other.
X1 Professional Client has the best following features: indexing engine, files previewer, customization, extensibility, efficiency for indexing and retrieving data, search operators and filters. The only missing feature, always requested by users over years but never implemented, is indexing Web Surfing History. Indexing your Web browser cache is a poor workaround but it works. I have been using Seruku IE Toolbar while surfing with Internet Explorer but this really good utility has been discontinued and was never implemented for firefox. One day I will give a try to addons ScrapBook, ScrapBook Plus or even Evernote. Everything would not do the trick as it does not idex file contents but filenames only

And my purpose is to have index and search results on my PC, not on any server of any ads seller!
Cheers from France