Depends on what language you need to support.

For python, elpy pretty much "just works" (some kicking required). It has a built-in diagnostic helper that will tell you what packages you're missing.

For anything supported by clang, company-clang or clangd work quite well.

For everything else, rtags/gtags or similar is the way to go. It uses a superficial understanding of the files, so black magic (e.g. putting `int=str` at the top of your python file) will confuse it, but it's generally good enough.