What does Mashkov mean by the greatest place on Earth?

It's hard to say without context - Mashkov (I'm assuming you mean Vladimir Mashkov) could've said something like that in an interview, a film, or on social media, but you'd need to know exactly where and when he said it. Try searching for the full quote or clarify which interview or project this phrase is from - then it'll be clear whether he's talking about an actual place on the map or something more philosophical.

Most likely he's talking about Lake Baikal - Mashkov has indeed repeatedly expressed enthusiasm about this lake and filmed there. But here's what's important: actors often give interviews to various publications and say similar things with minor variations, so without a specific reference to the exact interview or film, it's hard to understand in what context exactly he called the place "greatest" - it could have just been a poetic description in one interview, while in another he expressed himself differently.

You'd need to specify the exact source of the quote - an interview, a film, social media, so we're not just guessing 😊 Mashkov really does talk enthusiastically about Lake Baikal and has filmed there, but without the exact quote you might end up with a paraphrase of his words where the meaning got a bit twisted or it got mixed up with something another actor said.