Like any "best" question, there's no real answer to this: "best" anything is an opinion, nothing more.
That said, there are some excellent tools around for people who struggle with grammar, whether it's because they're dyslexic, English isn't their first language or they're simply more interested in being creative and don't have time to fart around with semicolons and ellipses.
Grammarly, Hemingway, Ginger, MS Word's built-in checkers... they're all good. They have their bad moments, of course, like this one kindly supplied by Abbas:
But "best"? That will always be a human editor.
No tool can spot context consistently, no tool can spot rhythm and flow, no tool will fact-check for you, no tool can help you restructure an article so it leads your reader from start to finish.
If you want a tool to spot spelling mistakes and grammar oopsies, any of those named above (or others) will suffice. If you want to write better content, build your reputation and start charging more, get an editor.
Can't afford one? Include their fee in your charges, so they cost you nothing and the client gets great content. Simple.